BGP

When peer tracking is enabled on a session, the neighbor IP address is tracked in the routing table. If a failure occurs and there is no longer any IP route matching the neighbor address, or else if the longest prefix match (LPM) route is rejected by the configurable peer-tracking-policy, then after a 1-second delay the session is taken down. By default, peer-tracking is disabled on all sessions. The default peer-tracking policy allows any type of route to match the neighbor IP address, except aggregate routes and LDP shortcut routes.

Peer tracking was introduced when BFD was not yet supported for peer failure detection. Now that BFD is available, peer-tracking has less value and is used less often.

SR OS also supports the option to setup an async-mode Bidirectional Forwarding Detection (BFD) session to a BGP neighbor so that failure of the BFD session can trigger immediate teardown of the BGP session. When BFD is enabled on a BGP session a 1-hop or multi-hop BFD session is setup to the neighbor IP address and the BFD parameters come from the BFD configuration of the interface associated with the local-address; for multi-hop sessions this is typically the system interface. With a 10 ms transmit interval and a multiplier of 3 BFD can detect a peer failure in a period of time as short of 30 ms.

Fast external failover applies only to single-hop EBGP sessions. When fast external failover is enabled on a single-hop EBGP session and the interface associated with the session goes down the BGP session is immediately taken down as well, even if other mechanisms such as the hold-timer have not yet indicated a failure.

Some BGP routers do not have redundant control plane processor modules or do not support BGP HA with the same quality or coverage as 7450 ESS, 7750 SR, or 7950 XRS routes. When dealing with such routers or specific error conditions, BGP graceful restart (GR) is a good option for minimizing the network disruption caused by a control plane reset.

BGP GR assumes that the router restarting its BGP sessions has the ability and architecture to continue packet forwarding throughout the control plane reset. If this is the case, then the peers of the restarting router act as helpers and ‟hide” the control plane reset from the rest of the network so that forwarding can continue uninterrupted. Forwarding based on stale routes and hiding the ‟staleness” from other routers is considered acceptable because the duration of the control plane outage is expected to be relatively short (a few minutes). For BGP GR to be used on a session, both routers must advertise the BGP GR capability during the OPEN message exchange; see the BGP advertisement section for more details.

BGP GR is enabled on one or more BGP sessions by configuring the graceful-restart command in the global, group, or neighbor context. The command causes GR mode to be supported for the following active families:

• IPv4 unicast

• IPv6 unicast

• VPN-IPv4

• VPN-IPv6

• label-IPv4

• label-IPv6

• L2-VPN

• route-target (RTC)

• flow-IPv4 (IPv4 FlowSpec)

• flow-IPv6 (IPv6 FlowSpec)

Helper mode is activated when one of the following events affects an 'Established' session:

• TCP socket error

• new inbound TCP connection from the peer

• hold timer expiry

• peer unreachable

• BFD down

• sent NOTIFICATION message (only if enable-notification is configured under graceful-restart and the peer set the ‟N” bit in its GR capability, and the NOTIFICATION is not a 'Cease' with subcode 'Hard Reset')

• received NOTIFICATION message (only if enable-notification is configured under graceful-restart and the peer set the ‟N” bit in its GR capability, and the NOTIFICATION is not a 'Cease' with subcode 'Hard Reset')

As soon as the failure is detected, the helping 7450 ESS, 7750 SR, or 7950 XRS router marks all the routes received from the peer as stale and starts a restart timer. The stale state is not factored into the BGP decision process, and it is not made visible to other routers in the network. The restart timer derives its initial value from the Restart Time carried in the last GR capability of the peer. The default advertised Restart Time is 300 seconds, but it can be changed using the restart-time command.

When the restart timer expires, helping stops if the session is not yet re-established. If the session is re-established before the restart timer expires and the new GR capability from the restarting router indicates that the forwarding state has been preserved, then helping continues and the peers exchange routes per the normal procedure.

When each router has advertised all its routes for a specific address family, it sends an End-of-RIB marker (EOR) for the address family. The EOR is a minimal UPDATE message with no reachable or unreachable Network Layer Reachability Information (NLRI) for the AFI or SAFI. When the helping router receives an EOR, it deletes all remaining stale routes of the AFI or SAFI that were not refreshed in the most recent set of UPDATE messages. The maximum amount of time that routes can remain stale (before being deleted if they are not refreshed) is configurable using the stale-routes-time.

SR OS supports Long-Lived Graceful Restart (LLGR). LLGR is supported for the same address families as normal GR, as described in BGP graceful restart.

The LLGR procedures adhere to draft-uttaro-idr-bgp-persistence-03. LLGR is intended to handle more serious and longer-term outages than ordinary GR.

SR OS routers support LLGR in the context of both the restarting router (which experienced a restart or failure) and the helper or receiving router (which is a peer of the failed router). Both functionalities are enabled and disabled at the same time by adding the long-lived command under a graceful-restart configuration context.

When long-lived is applied to a session (and capability negotiation is not disabled), the OPEN message sent to the peer includes both the GR capability and the LLGR capability. Both capabilities list the same set of AFI/SAFI.

The operation of a network can be compromised if an unauthorized system is able to form or hijack a BGP session and inject control packets by falsely representing itself as a valid neighbor. This risk can be mitigated by enabling TCP MD5 authentication on one or more of the sessions. When TCP MD5 authentication is enabled on a session every TCP segment exchanged with the peer includes a TCP option (19) containing a 16-byte MD5 digest of the segment (more specifically the TCP/IP pseudo-header, TCP header and TCP data). The MD5 digest is generated and validated using an authentication key that must be known to both sides. If the received digest value is different from the locally computed one then the TCP segment is dropped, thereby protecting the router from spoofed TCP segments.

The TTL security mechanism (GTSM) relies on a simple concept to protect BGP infrastructure from spoofed IP packets. It recognizes the fact that the vast majority of EBGP sessions are established between directly-connected routers and therefore the IP TTL values in packets belonging to these sessions should have predictable values. If an incoming packet does not have the expected IP TTL value it is possible that it is coming from an unauthorized and potentially harmful source.

TTL security is enabled using the ttl-security command. This command requires a minimum TTL value to be specified. When TTL security is enabled on a BGP session the IP TTL values in packets that are supposedly coming from the peer are compared (in hardware) to the configured minimum value and if there is a discrepancy the packet is discarded and a log is generated. TTL security is used most often on single-hop EBGP sessions but it can be used on multihop EBGP and IBGP sessions as well.

To enable TTL security on a single-hop EBGP session, configure ttl-security and multihop to a value of 255. To enable TTL security on a multihop EBGP session, configure ttl-security and multihop to match the expected TTL of (255 - hop count). The TTL value for both EBGP peers must be manually configured to the same value, as there is no TTL negotiation.

If the AS number is changed at the router level (config>router) the new AS number is not used until the BGP instance is restarted either by administratively disabling and enabling the BGP instance or by rebooting the system with the new configuration.

On the other hand, if the AS number is changed in the BGP configuration (config>router>bgp), the effects are as follows.

• A change of the local-AS at the global level causes the BGP instance to restart with the new local AS number.

• A change of the local-AS at the group level causes BGP to re-establish sessions with all peers in the group using the new local AS number.

• A change of the local-AS at the neighbor level causes BGP to re-establish the session with the new local AS number.

Changing the confederation value on an active BGP instance does not restart the protocol. The change takes effect when the BGP protocol is (re) initialized.

BGP advertisement allows a BGP router to indicate to a peer, using the optional parameter, the features that it supports so that they can coordinate and use only the features that both support. Each capability in the optional parameter is TLV-encoded with a unique type code. SR OS supports the following capability codes:

• multi-protocol BGP (code 1)

• route refresh (code 2)

• outbound route filtering (code 3)

• graceful restart (code 64)

• 4-octet AS number (code 65)

• add-path (code 69)

The approach to handling update message errors has evolved in the past couple of years. The original BGP protocol specification called for all update message errors to be handled the same way (that is, send a notification to the peer and immediately close the BGP session). This error handling approach was motivated by the goal to ensure protocol ‟correctness” above all else. But, it ignored several important points.

• Not all UPDATE message errors truly have the same severity. If the NLRI cannot be extracted and parsed from an UPDATE message then this is indeed a ‟critical” error. But other errors such as incorrect attribute flag settings, missing mandatory path attributes, incorrect next-hop length/format, and so on. can be considered ‟non-critical” and handled differently.

• Session resets are extremely costly in terms of their impact on the stability and performance of the network. For many types of UPDATE message errors, a session reset does not solve the problem because the root cause remains (for example, software error, hardware error or misconfiguration). If a session reset is absolutely necessary, then the operator should have some control over the timing.

• Some degree of protocol ‟incorrectness” is tolerable for a short period of time as long as the network operator is fully aware of the issue. In this context, ‟incorrectness” typically means a BGP RIB inconsistency between routers in the same AS. Such inconsistency has become less and less of an issue over time as edge-to-edge tunneling of IP traffic (for example, BGP shortcuts, IP VPN) has reduced the number of deployments where IP traffic is forwarded hop-by-hop.

In recognition of these points and the general trend toward more flexibility in BGP error handling, SR OS supports a BGP configuration option called update-fault-tolerance that allows the operator to decide whether the router should apply new or legacy error handling procedures to update message errors. If update-fault-tolerance is configured, then non-critical errors as described above are handled using the ‟treat-as-withdraw” or ‟attribute-discard” approaches to error handling; these approaches do not cause a session reset. If update-fault-tolerance is not configured then legacy procedures continue to apply and all errors (critical and non-critical) trigger a session reset.

If the update-fault-tolerance command was previously configured and a non-critical error was already triggered, the BGP session is still reset when the operator configures no update-fault-tolerance.

The AS override feature can be used in VPRN scenarios where a customer is running BGP as the PE-CE protocol and some or all of the CE locations are in the same Autonomous System (AS). With normal BGP, two sites in the same AS would not be able to reach each other directly because there is an apparent loop in the AS path.

When override is configured on a PE-CE EBGP session the PE rewrites the customer ASN in the AS path with the VPRN AS number as the route is advertised to the CE.

The description in the previous section does fully describe the reasons for using local-as. This BGP feature facilitates the process of changing the ASN of all the routers in a network from one number to another. This may be necessary if one network operator merges with or acquires another network operator and the two BGP networks must be consolidated into one autonomous system.

For example, suppose the operator of the ASN 64500 network merges with the operator of the ASN 64501 network and the new merged entity decides to renumber ASN 64501 routers as ASN 64500 routers, so that the entire network can be managed as one autonomous system. The migration can be carried out using the following sequence of steps.

1. Change the global AS of the route reflectors that used to be part of ASN 64501 to the new value 64500.

2. Change the global AS of the RR clients that used to be part of ASN 64501 to the new value 64500.

3. Configure local-as 64501 private no-prepend-global-as on every EBGP session of each RR client migrated in step 2.

This migration procedure has several advantages. First, customers, settlement-free peers and transit providers of the previous ASN 64501 network still perceive that they are peering with ASN 64501 and can delay switching to ASN 64500 until the time is convenient for them. Second, the AS path lengths of the routes exchanged with the EBGP peers are unchanged from before so that best path selections are preserved.

When BGP was developed, it was assumed that 16-bit (2-octet) ASNs would be sufficient for global Internet routing. In theory a 16-bit ASN allows for 65536 unique autonomous systems but some of the values are reserved (0 and 64000-65535). Of the assignable space less than 10% remains available. When a new AS number is needed it is now simpler to obtain a 4-octet AS number. 4-octet AS numbers have been available since 2006. A 32-bit (4-octet) ASN allows for 4,294,967,296 unique values (some of which are again, reserved).

When 4-octet AS numbers became available it was recognized that not all routers would immediately support the ability to parse 4-octet AS numbers in BGP messages so two optional transitive attributes called AS4_PATH and AS4_AGGREGATOR were introduced to allow a gradual migration.

A BGP router that supports 4-octet AS numbers advertises this capability in its OPEN message; the capability information includes the AS number of the sending BGP router, encoded using 4 bytes (recall the ASN field in the OPEN message is limited to 2 bytes). By default, OPEN messages sent by 7450, 7750, or 7950 routers always include the 4-octet ASN capability, but this can changed using the disable-4byte-asn command.

If a BGP router and its peer have both announced the 4-octet ASN capability, then the AS numbers in the AS_PATH and AGGREGATOR attributes are always encoded as 4-byte values in the UPDATE messages they send to each other. These UPDATE messages should not contain the AS4_PATH and AS4_AGGREGATOR path attributes.

If one of the routers involved in a session announces the 4-octet ASN capability and the other one does not, then the AS numbers in the AS_PATH and AGGREGATOR attributes are encoded as 2-byte values in the UPDATE messages they send to each other.

When a 7450, 7750, or 7950 router advertises a route to a peer that did not announce the 4-octet ASN capability.

• If there are any AS numbers in the AS_PATH attribute that cannot be represented using 2 bytes (because they have a value greater than 65535) they are substituted with the special value 23456 (AS_TRANS) and an AS4_PATH attribute is added to the route if it is not already present. The AS4_PATH attribute has the same encoding as the AS_PATH attribute that would be sent to a 4-octet ASN capable router (that is, each AS number is encoded using 4 octets) but it does not carry segments of type AS_CONFED_SEQUENCE or AS_CONFED_SET.

• If the AS number in the AGGREGATOR attribute cannot be represented using 2 bytes (because its value is greater than 65535) it is substituted with the special value 23456 and as AS4_AGGREGATOR attribute is added to the route if it is not already present. The AS4_AGGREGATOR is the same as the AGGREGATOR attribute that would be sent to a 4-octet ASN capable router (that is, the AS number is encoded using 4 octets).

When a 7450, 7750, or 7950 router receives a route with an AS4_PATH attribute it attempts to reconstruct the full AS path from the AS4_PATH and AS_PATH attributes, regardless of whether disable-4byte-asn is configured or not. The reconstructed path is the AS path displayed in BGP show commands. If the length of the received AS4_PATH is N and the length of the received AS_PATH is N+t, then the reconstructed AS path contains the t leading elements of the AS_PATH followed by all the elements in the AS4_PATH.

By default, IPv4 routes are advertised with IPv4 next-hops but on IPv6-TCP transport sessions they can be advertised with IPv6 next-hops if the advertise-ipv6-next-hops command (with the IPv4 option) applies to the session. To receive IPv4 routes with IPv6 next-hop addresses from a peer, the extended-nh-encoding command (with the IPv4 option) must be applied to the session. This advertises the corresponding RFC 5549, Advertising IPv4 Network Layer Reachability Information with an IPv6 Next Hop, capability to the peer.

Whenever next-hop-self applies to an IPv4 route, the next hop is set as follows.

• If the peer whose routes are being advertised is an IPv4 transport peer (in other words, the neighbor address is IPv4), the BGP next-hop is the IPv4 local address used to setup the session.

• If the peer toward which the routes are being advertised is an IPv6 transport peer (in other words, the neighbor address is IPv6), and the advertise-ipv6-next-hops command (with the ipv4 option) applies to the session, and the peer toward which the routes are being advertised opened the session by announcing an Extended NH Encoding capability for AFI =1, SAFI = 1 and next-hop AFI = 2, then the BGP next-hop is the IPv6 local address used to setup the session. Otherwise, for all other cases, the BGP next-hop is the IPv4 address of the system interface. If the system interface does not have an IPv4 address, the route is not advertised unless an export policy sets a valid IPv4 next-hop.

When an IPv4 BGP route is advertised to an EBGP peer, next-hop-self always applies except if the third-party-nexthop command is applied. Configuring third-party-nexthop allows an IPv4 route received from one EBGP peer to be advertised to another EBGP that is in the same IP subnet with an unchanged BGP next-hop.

When an IPv4 BGP route is re-advertised to an IBGP or confederation EBGP peer, the advertising router does not modify the BGP next-hop unless one of the following applies.

• The BGP next-hop-self command is applied to the IBGP or confederation EBGP peer. This causes next-hop-self to be applied to all IPv4 routes advertised to the peer, regardless of the peer type from which they were received (IBGP, confed-EBGP, or EBGP).

• IPv4 routes are matched and accepted by a route policy entry, and this entry has a next-hop-self action. This applies next-hop-self as described above to only those routes matched by the policy entry.

• IPv4 routes are matched and accepted by route policy entry, and this entry has a next-hop ip-address action. This changes the BGP next-hop of only the matched routes to the ip-address, if the ip-address is an IPv4 address or if the ip-address is an IPv6 address and the necessary conditions exist. The advertise-ipv6-next-hops command is configured appropriately and the peer opened the session with the correct RFC 5549 capability.

When an IPv4 BGP route is locally originated and advertised to an IBGP or confederation EBGP peer, the BGP next-hop is, by default, copied from the next hop of the route that was imported into BGP, with specified exceptions (for example, black-hole next-hop). When a static route with indirect next hop is re-advertised as a BGP route, the BGP next-hop is a copy of the indirect address. However, with route table import policies, BGP can be instructed to take the resolved next hop of the static route as the BGP next-hop address.

SR OS routers never send or receive IPv6 routes with 32-bit IPv4 next-hop addresses.

When an IPv6 BGP route is advertised to an EBGP peer, next-hop-self always applies (except if the third-party-nexthop command is applied, as described in the following note). Next-hop-self results in one of the following outcomes.

• If the EBGP session uses IPv4 transport, the BGP next-hop encodes the local-address used for setup of the session as an IPv4-compatible IPv6 address (all zeros in the first 96 bits followed by the 32 bit IPv4 local-address).

• If the EBGP session uses IPv6 transport, the BGP next-hop is the local-address used to setup the session and this cannot be overridden, even by BGP export policy.

When an IPv6 BGP route is re-advertised to an IBGP or confederation-EBGP peer, the advertising router does not modify the BGP next-hop by default; however, this can be changed as follows.

• If the BGP next-hop-self command is applied to the IBGP peer or confederation-EBGP peer, then this changes the BGP next-hop to the local-address used to setup the session (if the transport to the peer is IPv6) or to an IPv4-compatible IPv6 address derived from the IPv4 local-address used to setup the session (if the transport to the peer is IPv4). This command applies to all routes advertised to the peer, regardless of the peer type from which they were received (IBGP, confed-EBGP, or EBGP).

• If IPv6 routes are matched and accepted by an export policy applied to an IBGP or confederation-EBGP session, and the matching policy entry has a next-hop-self action, this changes the BGP next-hop of only the matched routes to the local-address used to setup the session (if the transport to the peer is IPv6) or to an IPv4-compatible IPv6 address derived from the IPv4 local-address used to setup the session (if the transport to the peer is IPv4).

• If IPv6 routes are matched and accepted by an export policy applied to an IBGP or confederation-EBGP session, and the matching policy entry has a next-hop <ip-address> action, this changes the BGP next-hop of only the matched routes to <ip-address>, but only if <ip-address> is an IPv6 address. If <ip-address> is an IPv4 address the matched routes are treated as though they were rejected by the policy entry.

When an IPv6 BGP route is locally originated and advertised to an IBGP or confederation- EBGP peer, the BGP next-hop is, by default, copied from the next-hop of the route that was imported into BGP, with specified exceptions (for example, black-hole next-hop). When a static route with indirect next-hop is re-advertised as a BGP route, the BGP next-hop is a copy of the indirect address, however with route-table-import policies BGP can be instructed to take the resolved next-hop of the static route as the BGP next-hop address.

SR OS routers can send and receive VPN-IPv4 routes with IPv4 next-hops. They can also be configured (using the extended-nh-encoding command) to receive VPN-IPv4 routes with IPv6 next-hop addresses from selected BGP peers by signaling the corresponding Extended NH Encoding BGP capability to those peers during session setup. If the capability is not advertised to a peer, such routes are not accepted from that peer. Also, if the SR OS router does not receive an Extended NH Encoding capability advertisement for [NLRI AFI=1, NLRI SAFI=128, next-hop AFI=2] from a peer, it does not advertise VPN-IPv4 routes with IPv6 next-hops to that peer.

When a VPN-IPv4 BGP route is advertised to an EBGP peer, the next-hop-self command applies if the enable-inter-as-vpn command is configured or if the enable-subconfed-vpn-forwarding and the next-hop-self commands are configured toward the EBGP peer; otherwise there is no change to the next-hop. The next-hop-self command results in one of the following outcomes.

• If the EBGP session uses IPv4 transport, the BGP next-hop is taken from the value of the local address used to set up the session.

• If the EBGP peer has opened an IPv6-transport session by advertising an extended NH encoding capability with (NLRI AFI=1, NLRI SAFI=128, next-hop AFI=2) and, in the configuration of the local router, the session is associated with an advertise-ipv6-next-hops vpn-ipv4 command, then the BGP next-hop is set to the value of the IPv6 local address used to set up the session. Otherwise, the BGP next-hop is set to the IPv4 address of the system interface.

If the enable-inter-as-vpn command is configured, the next-hop-self command applies automatically if a VPN-IPv4 BGP route is received from an EBGP peer and readvertised to an IBGP or confederation-EBGP peer. If the enable-inter-as-vpn command is not configured but the enable-subconfed-vpn-forwarding command is configured, the next-hop-self command can be set manually toward any IBGP or confederation-EBGP peer with the same label-swap forwarding behavior as provided by the enable-inter-as-vpn command. In either case, the next-hop-self command results in one of the following outcomes.

• If the IBGP or confederation-EBGP session uses IPv4 transport, then the BGP next-hop is taken from the value of the local address used to set up the session.

• If the IBGP or confederation-EBGP peer has opened an IPv6-transport session by advertising an extended NH encoding capability with (NLRI AFI=1, NLRI SAFI=128, next-hop AFI=2) and, in the configuration of the local router, the session is associated with an advertise-ipv6-next-hops vpn-ipv4 command, then the BGP next-hop is set to the value of the IPv6 local address used to set up the session. Otherwise, the BGP next-hop is set to the IPv4 address of the system interface.

When a VPN-IPv4 BGP route is reflected from one IBGP peer to another IBGP peer, the RR does not modify the next-hop by default. However, if the next-hop-self command is applied to the IBGP peer receiving the route and the enable-rr-vpn-forwarding command is configured, a next-hop-self and label swap behavior can be provided. Alternatively, the same behavior can be achieved by configuring the enable-subconfed-vpn-forwarding command and setting the next-hop-self command toward the targeted IBGP peer. In either case, next-hop-self results in one of the following outcomes.

• If the IBGP session receiving the reflected route uses IPv4 transport, the BGP next-hop is taken from the value of the local address used to set up the session.

• If the IBGP session receiving the reflected route has opened an IPv6-transport session by advertising an extended NH encoding capability with (NLRI AFI=1, NLRI SAFI=128, next-hop AFI=2) and, in the configuration of the local router, the session is associated with an advertise-ipv6-next-hops vpn-ipv4 command, then the BGP next-hop is set to the value of the IPv6 local address used to set up the session. Otherwise, the BGP next-hop is set to the IPv4 address of the system interface.

When a VPN-IPv4 BGP route is reflected from one IBGP peer to another IBGP peer and enable-rr-vpn-forwarding command is configured and the VPN-IPv4 route is matched and accepted by an export policy entry with a next-hop <ip-address> action, this changes the BGP next-hop of the matched routes to <ip-address>, except if <ip-address> is an IPv6 address and the receiving IBGP peer did not advertise an extended NH encoding capability with (NLRI AFI=1, NLRI SAFI=128, next-hop AFI=2) or, in the configuration of the local router, the session is not associated with an advertise-ipv6-next-hops vpn-ipv4 command. In this case, the route is treated as though it was rejected by the policy entry.

SR OS routers never send or receive VPN-IPv6 routes with 32-bit IPv4 next-hop addresses.

When a VPN-IPv6 BGP route is advertised to an EBGP peer, the next-hop-self command applies if the enable-inter-as-vpn command is configured or if the enable-subconfed-vpn-forwarding and the next-hop-self commands are configured toward the EBGP peer. Otherwise, there is no change to the next-hop. The next-hop-self command results in one of the following outcomes.

• If the EBGP session uses IPv4 transport, then the BGP next-hop is set to the IPv4 local address used to set up the session but encoded as an IPv4-mapped IPv6 address (for example, with the IPv4 address in the least significant 32 bits of a ::FFFF/96 prefix).

• If the EBGP session uses IPv6 transport and it is associated with an advertise-ipv6-next-hops vpn-ipv6 command, the BGP next-hop is set to the value of the IPv6 local address used to set up the session. Otherwise, the BGP next-hop is set to the IPv4 address of the system interface encoded as an IPv4-mapped IPv6 address (for example, with the IPv4 address in the least significant 32 bits of a ::FFFF/96 prefix).

If the enable-inter-as-vpn command is configured, the next-hop-self command applies automatically if a VPN-IPv6 BGP route is received from an EBGP peer and re-advertised to an IBGP or confederation-EBGP peer. If the enable-inter-as-vpn command is not configured but the enable-subconfed-vpn-forwarding command is configured, the next-hop can be set manually toward any IBGP or confederation EBGP peer, with the same label-swap forwarding behavior as provided by the enable-inter-as-vpn command. In either case, the next-hop-self command results in one of the following outcomes.

• If the IBGP or confederation EBGP session uses IPv4 transport, then the BGP next-hop is set to the IPv4 local address used to set up the session but encoded as an IPv4-mapped IPv6 address (for example, with the IPv4 address in the least significant 32 bits of a ::FFFF/96 prefix).

• If the IBGP or confederation EBGP session uses IPv6 transport and it is associated with an advertise-ipv6-next-hops vpn-ipv6 command, then the BGP next-hop is set to the value of the IPv6 local address used to set up the session. Otherwise, the BGP next-hop is set to the IPv4 address of the system interface encoded as an IPv4-mapped IPv6 address (for example, with the IPv4 address in the least significant 32 bits of a ::FFFF/96 prefix).

When a VPN-IPv6 BGP route is reflected from one IBGP peer to another IBGP peer, the RR does not modify the next-hop by default. However, if the next-hop-self command is applied to the IBGP peer receiving the route and the enable-rr-vpn-forwarding command is configured, a next-hop-self command and label swap behavior can be provided. Alternatively, the same behavior can be achieved by configuring the enable-subconfed-vpn-forwarding command and setting the next-hop-self command toward the targeted IBGP peer. In either case, the next-hop-self command results in one of the following outcomes.

• If the IBGP session receiving the reflected route uses IPv4 transport then the BGP next-hop is set to the IPv4 local-address used to set up the session but encoded as an IPv4-mapped IPv6 address (for example, with the IPv4 address in the least significant 32 bits of a ::FFFF/96 prefix).

• If the IBGP session receiving the reflected route uses IPv6 transport and it is associated with an advertise-ipv6-next-hops vpn-ipv6 command, the BGP next-hop is set to the value of the IPv6 local address used for set up of the session. Otherwise, the BGP next-hop is set to the IPv4 address of the system interface encoded as an IPv4-mapped IPv6 address (for example, with the IPv4 address in the least significant 32 bits of a ::FFFF/96 prefix).

When a VPN-IPv6 BGP route is reflected from one IBGP peer to another IBGP peer, enable-rr-vpn-forwarding command is configured and the VPN-IPv6 route is matched and accepted by an export policy entry with a next-hop <ip-address> action, this changes the BGP next-hop of the matched routes to <ip-address> if it is specified as a 128-bit IPv6 address, or to an IPv4-mapped IPv6 address encoding <ip-address> if it is specified as a 32-bit IPv4 address.

SR OS routers can always send and receive label-IPv4 routes with IPv4 next-hops. They can also be configured (using the extended-nh-encoding command) to receive label-IPv4 routes with IPv6 next-hop addresses from selected BGP peers by signaling the corresponding Extended NH Encoding BGP capability to those peers during session setup. If the capability is not advertised to a peer then such routes are not accepted from that peer. Also, if the SR OS router does not receive an Extended NH Encoding capability advertisement for [NLRI AFI=1, NLRI SAFI=4, next-hop AFI=2] from a peer then it is not advertise label-IPv4 routes with IPv6 next-hops to that peer.

When a label-IPv4 BGP route is advertised to an EBGP peer, next-hop-self applies unless the EBGP session has next-hop-unchanged enabled for the label-ipv4 address family. Next-hop-self results in one of the following outcomes.

• If the EBGP session uses IPv4 transport, then the BGP next-hop is taken from the value of the local-address used to setup the session.

• If the EBGP peer opened an IPv6-transport session by advertising an extended NH encoding capability with (NLRI AFI=1, NLRI SAFI=4, next-hop AFI=2) AND, in the configuration of the local router, the session is associated with an advertise-ipv6-next-hops label-ipv4 command, then the BGP next-hop is set to the value of the IPv6 local-address used for setup of the session. Otherwise, the BGP next-hop is set to the IPv4 address of the system interface.

When a label-IPv4 BGP route is received from an EBGP peer and re-advertised to an IBGP or confederation-EBGP peer, next-hop-self applies unless the IBGP or confederation EBGP session has next-hop-unchanged enabled for the label-ipv4 address family. Next-hop-self results in one of the following outcomes.

• If the IBGP or confederation EBGP session uses IPv4 transport, then the BGP next-hop is taken from the value of the local-address used to setup the session.

• If the IBGP or confederation EBGP peer opened an IPv6-transport session by advertising an extended NH encoding capability with (NLRI AFI=1, NLRI SAFI=4, next-hop AFI=2) AND, in the configuration of the local router, the session is associated with an advertise-ipv6-next-hops label-ipv4 command then the BGP next-hop is set to the value of the IPv6 local-address used for setup of the session. Otherwise, the BGP next-hop is set to the IPv4 address of the system interface

When a label-IPv4 BGP route is reflected from one IBGP peer to another IBGP peer, the RR does not modify the next-hop by default. However, if the next-hop-self command is applied to the IBGP peer receiving the route, then this results in one of the following outcomes.

• If the IBGP session receiving the reflected route uses IPv4 transport, then the BGP next-hop is taken from the value of the local-address used to setup the session.

• If the IBGP session receiving the reflected route opened an IPv6-transport session by advertising an extended NH encoding capability with (NLRI AFI=1, NLRI SAFI=4, next-hop AFI=2) AND, in the configuration of the local router, the session is associated with an advertise-ipv6-next-hops label-ipv4 command then the BGP next-hop is set to the value of the IPv6 local-address used for setup of the session. Otherwise, the BGP next-hop is set to the IPv4 address of the system interface.

When a label-IPv4 BGP route is reflected from one IBGP peer to another IBGP peer and the label-IPv4 route is matched and accepted by an export policy entry with a next-hop <ip-address> action, this changes the BGP next-hop of the matched routes to <ip-address>, except if <ip-address> is an IPv6 address and the receiving IBGP peer did not advertise an extended NH encoding capability with (NLRI AFI=1, NLRI SAFI=128, next-hop AFI=2) or, in the configuration of the local router, the session is not associated with an advertise-ipv6-next-hops label-ipv4 command. In this case, the route is treated as though it was rejected by the policy entry.

SR OS routers never send or receive label-IPv6 routes with 32-bit IPv4 next-hop addresses.

When a label-IPv6 BGP route is advertised to an EBGP peer, next-hop-self command applies unless the EBGP session has next-hop-unchanged command enabled for the label-ipv6 address family. next-hop-self results in one of the following outcomes.

• If the EBGP session uses IPv4 transport, then the BGP next-hop is taken from the value of the local address used to set up the session and encoded as an IPv4-mapped IPv6 address.

• If the EBGP peer opened an IPv6 transport session and it is associated with an advertise-ipv6-next-hops label-ipv6 command then the BGP next-hop is set to the value of the IPv6 local address used for set up of the session. Otherwise, the BGP next-hop is set to the IPv4 address of the system interface encoded as an IPv4-mapped IPv6 address.

When a label-IPv6 BGP route is received from an EBGP peer and re-advertised to an IBGP or confederation EBGP peer, next-hop-self applies unless the IBGP or confederation EBGP session has the next-hop-unchanged command enabled for the label-IPv6 address family. The next-hop-self results in one of the following.

• If the IBGP or confederation EBGP session uses IPv4 transport, then the BGP next-hop is taken from the value of the local address used to setup the session and encoded as an IPv4-mapped IPv6 address.

• If the IBGP or confederation EBGP peer opened an IPv6 transport session and it is associated with an advertise-ipv6-next-hops label-ipv6 command then the BGP next-hop is set to the value of the IPv6 local address used for set up of the session. Otherwise, the BGP next-hop is set to the IPv4 address of the system interface encoded as an IPv4-mapped IPv6 address.

To use a BGP route for forwarding, a BGP router must know how to reach the BGP next-hop of the route. The process of determining the local interface or tunnel used to reach the BGP next-hop is called next-hop resolution. The BGP next-hop resolution process depends on the type of route (the AFI/SAFI) and various configuration settings.

• The following procedures apply to the next-hop resolution of IPv4 and IPv6 (unlabeled) unicast routes.

  • When an IPv6 route is received with an IPv4-mapped IPv6 address as next-hop, BGP next-hop resolution is based on the embedded IPv4 address and this IPv4 address is matched to IPv4 routes or IPv4 tunnels, depending on configuration. The IPv4-mapped IPv6 address is never interpreted as an IPv6 address to be compared to IPv6 routes or IPv6 tunnels.

  • BGP routes are eligible to resolve a BGP next-hop only if the use-bgp-routes command is configured. A maximum of four levels of recursion are supported.

  • If there are multiple eligible routes that match the BGP next-hop, the longest prefix match (LPM) route is selected.

  • If the LPM route is rejected by the user-configured next-hop-resolution policy or if there are no eligible matching routes, the BGP next-hop is unresolved and all the routes with that next-hop are considered invalid and not advertised to peers.

  • BGP shortcuts are described in Next-hop resolution of BGP unlabeled IPv4 unicast routes to tunnels.

• The following procedures apply to the next-hop resolution of VPN-IPv4 and VPN-IPv6 routes.

  • When an VPN-IPv4 or VPN-IPv6 route is received with an IPv4-mapped IPv6 address as next-hop, BGP next-hop resolution is based on the embedded IPv4 address and this IPv4 address is matched to IPv4 routes and/or IPv4 tunnels, depending on configuration. The IPv4-mapped IPv6 address is never interpreted as an IPv6 address to be compared to IPv6 routes or IPv6 tunnels.

  • If the VPN-IP route is imported into a VPRN, the next-hop is resolved to a tunnel based on the auto-bind-tunnel configuration of the importing VPRN. For more information, see the 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 3 Services Guide: IES and VPRN.

  • If the next-hop entry in the tunnel-table that resolves the VPN-IP route is rejected by the user-configured next-hop-resolution vpn-family-policy, the BGP next hop is unresolved and all the VPN-IP routes with that next hop are considered invalid.

  • If the VPN-IP route is received from an IBGP or EBGP peer, and the router is a next-hop-self RR or a model-B ASBR, the order of resolution is as follows:

    • local (interface) route

    • longest prefix-match static route, excluding default static routes, but only if allow-static is configured

    • tunnel, based on the transport-tunnel resolution-filter options for family VPN; for more information, see Next-hop resolution of BGP labeled routes to tunnels

• The following procedures apply to the next-hop resolution of EVPN routes.

  • If an EVPN ES route is imported, the next hop is resolved to any tunnel in the tunnel-table. If there is no entry matching the next hop in the tunnel-table, a route matching the next hop in the route-table also resolves the next hop.

    • If the resolving route in the tunnel-table or the route-table is rejected by a user-configured next-hop-resolution vpn-family-policy, the ES route's next hop is unresolved.

    • If the ES route next hop is unresolved, the PE that advertised the route is not considered as candidate for Designated Forwarder (DF) election.

  • The imported AD per-ES and AD per-EVI routes are always shown as resolved and valid, irrespective of the next-hop resolution or the configuration of a next-hop-resolution vpn-family-policy.

    • With DF election, the router always considers the advertising PE a valid candidate, even if the AD routes next hops are unresolved.

    • However, if the AD per-EVI next hop is unresolved, EVPN traffic is not sent to the advertising PE. This is true for EVPN VPWS or multi-homing aliasing or backup procedures.

    • A matching tunnel-table entry can resolve the next-hop of an AD per-EVI route in an EVPN-MPLS service. A route-table entry (other than a shortcut) can resolve the AD per-EVI route next-hop in an EVPN-VXLAN service.

  • For any other imported EVPN service route, including IP prefix, IPv6 prefix, MAC/IP, Inclusive Multicast (IMET) and Selective Multicast (SMET) routes, the next hop is resolved as follows.

    • In EVPN-MPLS services, the next hop is resolved to a tunnel based on the auto-bind-tunnel configuration of the importing service.

    • In EVPN-VXLAN services, the next hop is resolved to a route in the route-table. That route cannot be a shortcut route.

    • If the next-hop entry in the tunnel-table or route-table that resolves the EVPN service route is rejected by the user-configured next-hop-resolution vpn-family-policy, the BGP next hop is unresolved and all the EVPN routes with that next hop are considered invalid.

• The following procedures apply to the next-hop resolution of label-unicast IPv4 and label-unicast IPv6 routes.

  • When a BGP-LU route is received with an IPv4-mapped IPv6 address as the next-hop, BGP next-hop resolution is based on the embedded IPv4 address and this IPv4 address is matched to IPv4 routes or IPv4 tunnels, depending on the configuration. The IPv4-mapped IPv6 address is never interpreted as an IPv6 address to be compared to IPv6 routes or IPv6 tunnels.

  • If the BGP-LU route is received by a control-plane-only RR with the disable-route-table-install and rr-use-route-table commands configured, the order of resolution is as follows:

    • the local route

    • the longest prefix-match static route, excluding default static routes

      Use this route to resolve the BGP next-hop if it is a static route with blackhole next-hop. For other types of static routes, use this route to resolve the next-hop only if allow-static is configured. If it is another type of static route, use it to resolve the next-hop only if allow-static is configured.

    • the longest prefix-match IGP route

  • If a label-unicast IPv4 route or label-unicast IPv6 route with a label (other than explicit-null) is received from an IBGP or EBGP peer and the router is not an RR with the rr-use-route-table command configured, the order of resolution is as follows:

    • the local route

    • the longest prefix-match static route, excluding default static routes

      Use this route to resolve the BGP next-hop if it is a static route with a blackhole next-hop. If it is another type of static route, use it to resolve the next-hop only if allow-static is configured.

    • a tunnel, based on the transport-tunnel resolution-filter options for family label-ipv4 or label-ipv6 (depending on the situation); for more information, see Next-hop resolution of BGP labeled routes to tunnels

  • If a label-unicast IPv6 route with an IPv6 explicit-null label is received from an IBGP or EBGP peer and the router is not an RR with the rr-use-route-table command configured, the order of resolution is as follows:

    • the local route

    • the longest prefix-match static route, excluding default static routes

      Use this route to resolve the BGP next-hop if it is a static route with a blackhole next-hop. If it is another type of static route, use it to resolve the next-hop only if allow-static is configured

    • a tunnel, based on the transport-tunnel resolution-filter options for family label-ipv4; for more information, see Next-hop resolution of BGP labeled routes to tunnels

    • if configure>router>bgp>next-hop-resolution>labeled-routes>use-bgp-routes>label-ipv6-explicit-null is enabled and if the longest prefix length route that matches the next-hop is a BGP IPv4 unlabeled, BGP IPv6 unlabeled, or other 6PE route with an explicit-null label, then use that route, subject to the following conditions:

      the resolving route cannot be a leaked route

      an unlabeled IPv4 route or IPv6 route is ineligible to resolve the next-hop of a label-unicast IPv6 route if the unlabeled route has any of its own BGP next-hops resolved by an IGP route or a 6over4 route

      the label-unicast IPv6 route can be recursively resolved by other label-unicast IPv6 routes with explicit-null so that the final route has up to four levels of recursion

In SR OS next-hop resolution is not a one-time event. If the IP route or tunnel that was used to resolve a BGP next-hop is withdrawn because of a failure or configuration change an attempt is made to re-resolve the BGP next-hop using the next-best route or tunnel. If there are no more eligible routes or tunnels to resolve the BGP next-hop then the BGP next-hop becomes unresolved. The continual process of monitoring and reacting to resolving route/tunnel changes is called next-hop tracking. In SR OS next-hop tracking is completely event driven as opposed to timer driven; this provides the best possible convergence performance.

SR OS supports next-hop indirection for most types of BGP routes. Next-hop indirection means BGP next-hops are logically separated from resolved next-hops in the forwarding plane (IOMs). This separation allows routes that share the same BGP next-hops to be grouped so that when there is a change to the way a BGP next-hop is resolved only one forwarding plane update is needed, as opposed to one update for every route in the group. The convergence time after the next-hop resolution change is uniform and not linear with the number of prefixes; in other words, the next-hop indirection is a technology that supports Prefix Independent Convergence (PIC). SR OS uses next-hop indirection whenever possible; there is no option to disable the functionality.

The router supports the MPLS entropy label, as specified in RFC 6790, on RFC 8277 BGP labeled routes. LSR nodes in a network can load-balance labeled packets in a more granular way than by hashing on the standard label stack. For more information, see the 7450 ESS, 7750 SR, 7950 XRS, and VSR MPLS Guide.

Entropy Label Capability (ELC) signaling is not supported for labeled routes representing BGP tunnels. Instead, ELC is configured at the head end LER using the configure router bgp override-tunnel-elc command. This command causes the router to ignore any advertisements for ELC that may or may not be received from the network, and instead to assume that the whole domain supports entropy labels.

Deterministic MED is an optional enhancement to the BGP decision process that causes BGP to group paths that are equal up to the MED comparison step based on the neighbor AS. BGP compares the best path from each group to arrive at the overall best path. This change to the BGP decision process makes best path selection completely deterministic in all cases. Without deterministic-med, the overall best path selection is sometimes dependent on the order of route arrival because of the rule that MED cannot be compared in routes from different neighbor AS.

In a standard community, the first two bytes usually encode the AS number of the administrative entity that assigned the value in the last two bytes. In SR OS, a standard community value is configured using the format <asnum:comm-value>; the colon is a required separator character. In route policy applications, multiple standard community values can be matched with a regular expression in the format <regex1>:<regex2>, where regex1 and regex2 are two regular expressions that are evaluated one numerical digit at a time.

The following well-known standard communities are understood and acted upon accordingly by SR OS routers.

• NO_EXPORT

  When a route carries this community, it must not be advertised outside a confederation boundary (for example, to EBGP peers).

• NO_ADVERTISE

  When a route carries this community, it must not be advertised to any other BGP peer.

• NO_EXPORT_SUBCONFED

  When a route carries this community, it must not be advertised outside a member AS boundary (for example, to confed-EBGP peers or EBGP peers.

• LLGR_STALE

  When a route carries this community, it indicates that the route was propagated by a router that is a long-lived graceful restart helper and normally (in the absence of LLGR) the route would have been withdrawn.

• NO_LLGR

  When a route carries this community, it indicates that the route should not be retained and used past the normal graceful restart window of time.

• BLACKHOLE

  When a route carries this community, it indicates that the route should be installed into the FIB with a blackhole next-hop.

Standard communities can be added to or removed from BGP routes using BGP import and export policies. When a BGP route is locally originated by exporting a static or aggregate route into BGP, and the static or aggregate route has one or more standard communities, these community values are automatically added to the BGP route. This may affect the advertisement of the locally originated route if one of the well-known communities is associated with the static or aggregate route.

To remove all the standard communities from all routes advertised to a BGP peer, use the disable-communities standard command.

Extended communities serve specialized roles. Each extended community is eight bytes. The first one or two bytes identifies the type or sub-type and the remaining six or seven bytes identify a value. Some of the more common extended communities supported by SR OS include:

• Transitive 2-octet AS-specific

  • Route target (type 0x0002)

  • Route origin (type 0x0003)

  • OSPF domain ID (type 0x0005)

  • Source AS (type 0x0009)

  • L2VPN identifier (type 0x000A)

• Non-transitive 2-octet AS-specific

  Link bandwidth (0x4004)

• Transitive 4-octet AS-specific

  • Route target (type 0x0202)

  • Route origin (type 0x0203)

  • OSPF domain ID (type 0x0205)

  • Source AS (type 0x0209)

• Transitive IPv4-address-specific

  • Route target (type 0x0102)

  • Route origin (type 0x0103)

  • OSPF domain ID (type 0x0105)

  • L2VPN identifier (type 0x010A)

  • VRF route import (type 0x010B)

• Transitive opaque

  • OSPF route type (type 0x0306)

  • Color extended community (type 0x030B)

• Non-transitive opaque

  BGP origin validation state (type 0x4300)

• Transitive experimental

  • FlowSpec traffic rate (type 0x8006)

  • FlowSpec traffic action (type 0x8007)

  • FlowSpec redirect (type 0x8008)

  • FlowSpec traffic-remarking (0x8009)

  • Layer 2 info (type 0x800A)

• Transitive FlowSpec

  FlowSpec interface-set (type 0x0702)

• Non-transitive FlowSpec

  FlowSpec interface-set (type 0x4702)

Extended communities can be added to or removed from BGP routes using BGP import and export policies. When a BGP route is locally originated by exporting a static or aggregate route into BGP, and the static or aggregate route has one or more extended communities, these community values are automatically added to the BGP route.

To remove all the extended communities from all routes advertised to a BGP peer, use the disable-communities extended command.

Each large community is a 12-byte value, formed from the logical concatenation of three 4-octet values: a Global Administrator part, a Local Data part 1, and Local Data part 2. The Global Administrator is a four-octet namespace identifier, which should be an Autonomous System Number assigned by IANA. The Global Administrator field is intended to allow different Autonomous Systems to define large communities without collision. Local Data Part 1 is a four-octet operator-defined value and Local Data Part 2 is another four-octet operator-defined value.

In SR OS, a large community value is configured using the format <ext-asnum>:<ext-comm-val>:<ext-comm-val>; the colon is a required separator character between each of the 4-byte values. In route policy applications, it is possible to match multiple large community values with a regular expression in the format <regex1>&<regex2>&<regex3>, where regex1, regex2 and regex3 are three regular expressions, each evaluated one numerical (decimal) digit at a time.

Large communities can be added to or removed from BGP routes using BGP import and export policies. When a BGP route is locally originated by exporting a static or aggregate route into BGP, and the static or aggregate route has one or more large communities, these community values are automatically added to the BGP route.

To remove all the standard communities from all routes advertised to a BGP peer, use the disable-communities large command.

The import command is used to apply one or more policies (up to 15) to a neighbor, group or to the entire BGP context. The import command that is most-specific to a peer is the one that is applied. An import policy command applied at the neighbor level takes precedence over the same command applied at the group or global level. An import policy command applied at the group level takes precedence over the same command specified on the global level. The import policies applied at different levels are not cumulative. The policies listed in an import command are evaluated in the order in which they are specified.

When an IP route is rejected by an import policy it is still maintained in the RIB-IN so that a policy change can be made later on without requiring the peer to re-send all its RIB-OUT routes. This is sometimes called soft reconfiguration inbound and requires no special configuration in SR OS.

When a VPN route is rejected by an import policy or not imported by any services it is deleted from the RIB-IN. For VPN-IPv4 and VPN-IPv6 routes this behavior can be changed by configuring the mp-bgp-keep command; this option maintains rejected VPN-IP routes in the RIB-IN so that a Route Refresh message does not have to be issued when there is an import policy change.

When a BGP router has multiple paths in its RIB for the same NLRI, its BGP decision process is responsible for deciding which path is the best. The best path can be used by the local router and advertised to other BGP peers.

On 7450 ESS, 7750 SR, and 7950 XRS routers, the BGP decision process orders received paths based on the following sequence of comparisons. If there is a tie between paths at any step, BGP proceeds to the next step.

1. Select a valid route over an invalid route. If a BGP route is invalid because its next-hop it is not resolved then it may still be advertisable if there are no valid routes. For example, an unresolved route can be reflected by a route reflector if it is not trying to set next-hop-self.

2. Prefer a route for which disable-route-table-install does not apply over a route for which disable-route-table-install has been specified.

3. Prefer a non-stale route over a stale route (in the context of long-lived graceful restart).

4. A default route generated by the send-default command is less preferred than a default route programmed by other means.

5. Select the route with the lowest origin validation state, where Valid<Not-Found<Invalid.

6. Select the route with the numerically lowest route-table preference. For VPN-IP routes this also consider the number of VPRNs that imported the route.

7. Select the route with the highest local preference.

8. Select the route with an AIGP metric. If they both have an AIGP metric, select the route with the lowest sum of:

   1. AIGP metric value stored with the LOC-RIB copy of the route

   2. route-table or tunnel-table cost between the calculating router and the BGP next-hop in the received route

9. Select the route with the shortest AS path. AS numbers in AS_CONFED_SEQ and AS_CONFED_SET elements do not count toward the AS path length. This step is skipped if as-path-ignore is configured for the address family.

10. Select the route with the lowest Origin (IGP<EGP<Incomplete).

11. Select the route with the lowest MED. Only compare MED for non-imported routes that have the same neighbor AS by default. A missing MED attribute is considered equivalent to a MED value 0 by default. Defaults can be changed by using the always-compare-med command.

12. Select the route with the lowest owner type (BGP < BGP-label < BGP-VPN).

13. Prefer routes learned from EBGP peers over routes learned from IBGP and confed-EBGP peers.

14. Select the route with the lowest route-table or tunnel-table cost to the NEXT_HOP. This step is skipped if ignore-nh-metric is configured, or if the routes are associated with different RIBs. For VPN-IP routes received by a router without any configured VPRN services, next-hop cost is determined from the route-table cost.

15. Select the route with the lowest next-hop type. Resolutions made in the route table are preferred to resolutions made in the tunnel-table. This step is skipped if ignore-nh-metric is configured, or if the routes are associated with different RIBs.

16. Select the route received from the peer with the lowest router ID; this comes from the ORIGINATOR_ID attribute (if present) or the BGP identifier of the peer (received in its OPEN message). If ignore-router-id is configured, keep the current best path and skip the remaining steps.

17. Select the route with the shortest CLUSTER_LIST length.

18. Select the route received from the peer with the lowest IP address.

19. For VPN-IP routes imported into a VPRN, select the route with the lowest route-distinguisher value.

Each BGP RIB with IP routes (unlabeled IPv4, labeled-unicast IPv4, unlabeled IPv6, and labeled-unicast IPv6) submits its best path for each prefix to the common IP route table, unless the disable-route-table-install command is configured or the selective-label-ipv4-install command has prevented the installation. The best path is selected by the BGP decision process. The default preference for BGP routes submitted by the label-IPv4 and label-IPv6 RIBs (these appear in the route table and FIB as having a BGP-LABEL protocol type) can be modified by using the label-preference command. The default preference for BGP routes submitted by the unlabeled IPv4 and IPv6 RIBs can be modified by using the preference command.

If a BGP RIB has multiple BGP paths for the same IPv4 or IPv6 prefix that qualify as the best path up to a specific point in the comparison process, then a specified number of these multipaths can be submitted to the common IP route table. This is called BGP multipath and must be explicitly enabled using one or more commands in the multi-path context. These commands specify the maximum number of BGP paths, including the overall best path, that each BGP RIB can submit to the route table for any particular IPv4 or IPv6 prefix. If ECMP, with a limit of n, is enabled in the base router instance, then up to n paths are selected for installation in the IP FIB. In the data-path, traffic matching the IP route is load-shared across the ECMP next hops based on a per-packet hash calculation.

By default, the hashing is not sticky, meaning that when one or more of the ECMP BGP next hops fail, all traffic flows matching the route are potentially moved to new BGP next hops. If required, a BGP route can be marked (using the sticky-ecmp action in route policies) for sticky ECMP behavior so that BGP next hop failures are handled by moving only the affected traffic flows to the remaining next hops as evenly as possible. If new ECMP BGP next hops become available for a marked BGP, then route flows are moved as evenly as possible onto the resultant set of next hops. For more information about sticky ECMP, see BGP support for sticky ECMP.

A BGP route to an IPv4 or IPv6 prefix is a candidate for installation as an ECMP next hop only if it meets all of the following criteria:

• The multipath route must be the same type of route as the best path (same AFI/SAFI and, in some cases, same next-hop resolution method).

• The multipath route must tie with the best path for all criteria of greater significance than next-hop cost, except for criteria that are configured to be ignored.

• If the best path selection reaches the next-hop cost comparison, the multipath route must have the same next-hop cost as the best route, unless unequal-cost is configured.

• The multipath route must not have the same BGP next hop as the best path or any other multipath route.

• The multipath route must not cause the ECMP limit of the routing instance to be exceeded. Use the ecmp command to configure the ECMP limit of the routing instance.

• The multipath route must not cause the applicable maximum paths limit to be exceeded.

• The multipath route must have the same neighbor AS in its AS path as the best path if restrict is set to same-neighbor-as.

  By default, any path with the same AS path length as the best path, regardless of the neighbor AS, is eligible to be considered a multipath.

• The route must have the same AS path as the best path if restrict is set to exact-as-path.

  By default, any path with the same AS path length as the best path, regardless of the AS numbers, is eligible to be considered a multipath.

SR OS also supports IBGP multipath. In some topologies, a BGP next hop is resolved by an IP route that has multiple ECMP next hops. When ibgp-multipath is not configured, only one of the ECMP next hops is programmed as the next hop of the BGP route in the IOM. When ibgp-multipath is configured, the IOM attempts to use all the ECMP next hops of the resolving route in the forwarding state. Although the name of the ibgp-multipath command implies that it is specific to IBGP-learned routes, this is not the case. It also applies to routes learned from any multi-hop BGP session including routes learned from multi-hop EBGP peers.

The multi-path and ibgp-multipath commands are not mutually exclusive and work together. The first context enables ECMP load-sharing across different BGP next hops (corresponding to different BGP routes) while the ibgp-multipath enables ECMP load-sharing across the next hops of IP routes that resolve the BGP next hops.

Finally, ibgp-multipath does not control traffic load sharing toward a BGP next hop that is resolved by a tunnel, as when dealing with BGP shortcuts or labeled routes (VPN-IP, label-IPv4, or label-IPv6). When a BGP next hop is resolved by a tunnel that supports ECMP, the load-sharing of traffic across the ECMP next hops of the tunnel is automatic.

SR OS supports direct resolution of a BGP next hop to multiple RSVP-TE or SR-TE tunnels. In addition, a BGP next hop can be resolved by multiple LDP ECMP next hops that each correspond to a separate LDP-over-RSVP or LDP-over-SRTE tunnel. It is also possible for a BGP next hop to be resolved by an IGP shortcut route that has multiple RSVP-TE or SR-TE tunnels as its ECMP next hops.

Each sticky ECMP route uses 64 distribution buckets to apportion flows onto the available next hops. Sticky ECMP flow distribution as next hops are removed part 1, Sticky ECMP flow distribution as next hops are removed part 2, and Sticky ECMP flow distribution as next hops are removed part 3 provide an example of the distribution of flows over multiple BGP next hops as next hops are removed.

Sticky ECMP flow distribution as next hops are removed for 1.1.1.1/32 lists the sticky ECMP flow distribution as next hops are removed for 1.1.1.1/32.

Sticky ECMP flow distribution as next hops are added part 1, Sticky ECMP flow distribution as next hops are added part 2, and Sticky ECMP flow distribution as next hops are added part 3 provide an example of the distribution of flows over multiple BGP next hops as next hops are added.

The following table lists the sticky ECMP flow distribution as next hops are added for 1.1.1.1/32:

In some cases, the ECMP BGP next-hops of an IP route correspond to paths with very different bandwidths and it makes sense for the ECMP load-balancing algorithm to distribute traffic across the BGP next-hops in proportion to their relative bandwidths. The bandwidth associated with a path can be signaled to other BGP routers by including a link-bandwidth extended community in the BGP route. The link-bandwidth extended community is optional and non-transitive and encodes an autonomous system (AS) number and a bandwidth.

The SR OS implementation supports the link-bandwidth extended community in routes associated with the following address families: IPv4, IPv6, label-IPv4, label-IPv6, VPN-IPv4, and VPN-IPv6. The router automatically performs weighted ECMP for an IP BGP route when all of the ECMP BGP next-hops of the route include a link-bandwidth extended community. The relative weight of traffic sent to each BGP next-hop is visible in the output of the show router route-table extensive and show router fib extensive commands.

A route with a link-bandwidth extended community can be received from any IBGP peer. If such a route is received from an EBGP peer, the link-bandwidth extended community is stripped from the route unless an accept-from-ebgp command applies to that EBGP peer. However, a link-bandwidth extended community can be added to routes received from a directly connected (single hop) EBGP peer, potentially replacing the received Extended Community. This is accomplished using the add-to-received-ebgp command, which is available in group and neighbor configuration contexts.

When a route with a link-bandwidth extended community is advertised to an EBGP peer, the link-bandwidth extended community is removed by default. However, transitivity across an AS boundary can be allowed by configuring the send-to-ebgp command.

When a route with a link-bandwidth extended community is advertised to a peer using next-hop-self, the Extended Community is usually removed if it was not added locally (that is, by policy or add-to-received-ebgp command). However, in the special case that a route is readvertised (with next-hop-self) toward a peer covered by the scope of an aggregate-used-paths command, and the re-advertising router has installed multiple ECMP paths toward the destination each associated with a link-bandwidth extended community, the route is readvertised with a link-bandwidth extended community encoding the total bandwidth of all the used multi-paths.

The link-bandwidth extended community associated with a BGP route can be displayed using the show router bgp routes command. For the bandwidth value, the system automatically converts the binary value in the extended community to a decimal number in units of Mb/s (1 000 000 b/s).

Weighted ECMP across the BGP next-hops of an IP BGP route is supported in combination with ECMP at the level of the route or tunnel that resolves one or more of the ECMP BGP next-hops. This ECMP at the resolving level can also be weighted ECMP when the following conditions all apply:

• the BGP next-hop is resolved by an IP route (OSPF, IS-IS, or static) with MPLS LSP ECMP next-hops

• ibgp-multipath is configured under BGP

• config>router>weighted-ecmp is configured

Received label-unicast routes can be installed by BGP as tunnels in the tunnel table. In SR OS, the tunnel table is used to resolve a BGP next-hop to a tunnel when required by the configuration or the type of route (see Next-hop resolution). BGP tunnels play a key role in the following solutions:

• inter-AS model C

• carrier supporting carrier (CSC)

• seamless MPLS

BGP tunnels have a preference of 10 in the tunnel table, compared to 9 for LDP tunnels and 7 for RSVP tunnels. If the router configuration allows all types of tunnels to resolve a BGP next-hop, an RSVP LSP is preferred over an LDP tunnel, and an LDP tunnel is preferred over a BGP tunnel.

Further details about BGP-LU tunnels depending on the address family, are described below.

When a router is configured to perform next-hop-self for labeled unicast routes, the router must swap and advertise learned labeled unicast routes to downstream routers because there is no way for next-hop-self routers to know which BGP-LU routes are in use by downstream routers at a specific time. Additionally, a new service may resolve to a new labeled unicast route on a downstream router at any time. A next-hop-self router is required to readvertise all learned labeled routes downstream after best route selection.

A next-hop-self router may have local services that resolve to a subset of learned labeled unicast routes. In this case, it is possible to enable selective download of labeled unicast routes to datapath tables. Only the labeled unicast routes used by local services can be downloaded to the datapath.

Use the following command to install labeled unicast routes in local datapath tables that are used for NHLFE resolution by local services.

configure router bgp selective-label-ip no-install

This process is dynamic. When additional labeled unicast routes are called by services, they are automatically downloaded to the datapath tables. After a labeled unicast route is downloaded to a datapath, the route can also be used by IP shortcuts for next-hop resolution.

Use the following command to install labeled unicast routes to the RTM for NHLFE resolution by IP shortcuts and continue with selective install for local services. This option is used to extend the no-install mode via downloading all the labeled unicast routes to the RTM for IP next-hop resolution. The RTM is fully populated by labeled unicast routes, and services continue to trigger the download of only the required labeled unicast routes to the LTN.

configure router bgp selective-label-ip route-table-install-only

Labeled unicast routes required for NHLFE resolution are handled selectively. That is, local services that require resolution to a labeled unicast route trigger the download.

The selective-label-ip command does not impact how labeled unicast routes are readvertised downstream.

In the following figure, the 7750 SR performs next-hop-self for labeled unicast routes and readvertises all learned labeled unicast routes to access the network (such as ACC routers, including the 7250 IXR-e).

The following figure shows the logical view of the BGP-LU NHS component from the example in the preceding figure.

Enabling selective-label-ip install saves datapath table space that can be used by other applications.

BGP fast reroute is a feature that brings together indirection techniques in the forwarding plane and pre-computation of BGP backup paths in the control plane to support fast reroute of BGP traffic around unreachable/failed BGP next-hops. BGP fast reroute is supported with IPv4, label-IPv4, IPv6, label-IPv6, VPN-IPv4 and VPN-IPv6 routes. The scenarios supported by the base router BGP context are described in BGP fast reroute scenarios (base context).

See the VPRN section of the 7450 ESS, 7750 SR, 7950 XRS, and VSR Layer 3 Services Guide: IES and VPRN for more information about BGP fast reroute information specific to IP VPNs.

The QoS Policy Propagation through BGP (QPPB) is a feature that allows a QoS treatment (forwarding class and optionally priority) to be associated with a BGP IPv4, label-IPv4, IPv6, or label-IPv6 route that is installed in the routing table. This is done so that when traffic arrives on a QPPB-enabled IP interface and its source or destination IP address matches a BGP route with QoS information, the packet is handled according to the QoS of the matching route. SR OS supports QPPB on the following types of interfaces:

• base router network interfaces

• IES and VPRN SAP interfaces

• IES and VPRN spoke SDP interfaces

• IES and VPRN subscriber interfaces

QPPB is enabled on an interface using the qos-route-lookup command. There are separate commands for IPv4 and IPv6 so QPPB can be enabled in one mode, source, destination, or none, for IPv4 packets arriving on the interface, and a different mode source, destination, or none, for IPv6 packets arriving on the interface.

Different BGP routes for the same IP prefix can be associated with different QPPB information. If these BGP routes are combined in support of ECMP or BGP fast reroute then the QPPB information becomes next-hop specific. If these LOC-RIB routes are combined in support of ECMP or BGP fast reroute then the QPPB information becomes next-hop specific. This means that in destination QPPB mode the QoS assigned to a packet depends on the BGP next-hop that is selected for that particular packet by the ECMP hash or fast reroute algorithm. In source QPPB mode the QoS assigned to a packet comes from the first BGP next-hop of the IP route matching the source address.

Policy accounting is a feature that allows ‟classes” to be associated with specific IPv4 or IPv6 routes, static or BGP learned, when they are installed in the routing table. This is done for the following reasons.

• To collect ‟per-interface, per-class” traffic statistics on policy accounting-enabled interfaces of the router. This is supported by all FP2 and later generation cards and systems.

• To implement ‟per-interface, per-FP, per-class” traffic policing on policy accounting-enabled interfaces of the router. This is only supported for destination classes and only by FP4 cards and systems. The rate limit is applied per-interface, per-FP, per-class, when the IP interface is a distributed interface such as R-VPLS, LAG, or spoke SDP, that spans multiple complexes. Otherwise, for a simple interface, the rate limit is applied per-interface per-class.

For both applications the following IP interface types are supported:

• base router network interfaces

• IES and VPRN SAP interfaces

• IES and VPRN spoke SDP interfaces

• IES and VPRN subscriber interfaces (with some limitations)

• IES and VPRN R-VPLE interfaces

Policy accounting, and policing (if needed and supported), is enabled on an interface using the policy-accounting command. The name of a policy accounting template must be specified as an argument of this command. SR OS supports up to 1024 different templates. Each policy accounting template can have a list of source classes (up to 255), a list of destination classes (up to 255), and a list of policers (up to 63). Each source class, destination class, and policer, in their respective list, has an index number. Source class indexes and destination class indexes have a global meaning. In other words, destination-class index 5 in one template refers to the same set of routes as destination-class index 5 in another policy accounting template. Policer indexes have a local scope to the enclosing template. In one template, destination-class index 5 could use policer index 2 and in another template destination-class index 5 could use policer index 62. If a destination class has an associated policer then incoming traffic on each IP interface on which the template is applied is rate-limited based on that policer if the destination IP address matches a route with that destination class.

Policy accounting templates containing one or more source class identifiers cannot be applied to subscriber interfaces.

The policy accounting template tells the IOM the number of statistics and policer resources to use for each interface. These resources are derived from two pools that are sized per-FP. The first pool consists of policer statistics indexes. Every policy-accounting interface on a card or FP uses one of these resources for every source and destination class index listed in the template referenced by the interface. These are basic resources needed for statistics collection. The total reservation at the FP level is set using the configure card slot-number fp fp-number policy-accounting command.

The second pool (FP4 cards only) consists of policer index resources. Every policy-accounting interface on a complex uses one of these resources for every destination class associated with a policer in the template referenced by the interface. The total reservation of this second resource at the FP level is set using the configure card slot-number fp fp-number ingress policy-accounting policers command.

The total number of the above two resources, per FP, must be less than or equal to 128000. In addition, the second resource pool size must be less than or equal to the size of the first resource pool.

It is possible to increase or decrease the size of either resource sub-pool at any time. A decrease can cause some interfaces (randomly selected) to immediately lose their resources and stop counting or policing some traffic that was previously being counted or policed.

If the policy accounting is enabled on a spoke SDP or R-VPLS interface, all FPs in the system should have a reservation for each of the above resources, otherwise the show router interface policy-accounting command output reports that the statistics are possibly incomplete.

Through route policy or configuration mechanisms, a BGP or static route for an IP prefix can have a source class index (1 to 255), a destination class index (1 to 255) or both. When an ingress packet on a policy accounting-enabled interface [I1] is forwarded by the IOM and its destination address matches a BGP or static route with a destination class index [D], and [D] is listed in the relevant policy accounting template, then the packets-forwarded and IP-bytes-forwarded counters for [D] on interface [I1] are incremented accordingly. If [D] is also associated with a policer (FP4 only) the packet is also subjected to rate limiting as discussed above. The policer statistics displayed by the show router interface policy-accounting command include Layer 2 encapsulation and is different from the destination-class byte-level statistics.

When an ingress packet on a policy accounting-enabled interface [I2] is forwarded by the IOM and its source address matches a BGP or static route with a source class index [S], and [S] is listed in the relevant policy accounting template, the packets-forwarded and IP-bytes-forwarded counters for [S] on interface [I2] are incremented accordingly. Policing based on the source class is unsupported.

It is possible that different BGP or static routes for the same IP prefix (through different next hops) are associated with different class information. If these routes are combined in support of ECMP or fast reroute then the destination class of a packet depends on the next hop that is selected for that particular packet by the ECMP hash or fast reroute algorithm. If the source address of a packet matches a route with multiple next hops its source class is derived from the first next hop of the matching route.

Route Flap Damping (RFD) is a mechanism supported by 7450, 7750, and 7950 routers, as well as other BGP routers, that was designed to help improve the stability of Internet routing by mitigating the impact of route flaps. Route flaps describe a situation where a router alternately advertises a route as reachable and then unreachable or as reachable through one path and then another path in rapid succession. Route flaps can result from hardware errors, software errors, configuration errors, unreliable links, and so on. However not all perceived route flaps represent a true problem; when a best path is withdrawn the next-best path may not be immediately known and may trigger a number of intermediate best path selections (and corresponding advertisements) before it is found. These intermediate best path selections may travel at different speeds through different routers because of the effect of the min-route-advertisement interval (MRAI) and other factors. RFD does not handle this type of situation particularly well and for this and other reasons many Internet service providers do not use RFD.

In SR OS route flap damping is configurable; by default, it is disabled. It can be enabled on EBGP and confed-EBGP sessions by including the damping command in their group or neighbor configuration. The damping command has no effect on IBGP sessions. When a route of any type (any AFI/SAFI) is received on a non-IBGP session that has damping enabled.

• If the route changes from reachable to unreachable because of a withdrawal by the peer then damping history is created for the route (if it does not already exist) and in that history the Figure of Merit (FOM), an accumulated penalty value, is incremented by 1024.

• If a reachable route is updated by the peer with new path attribute values then the FOM is incremented by 1024.

• In SR OS the FOM has a hard upper limit of 21540 (not configurable).

• The FOM value is decayed exponentially as described in RFC 2439. The half-life of the decay is 15 minutes by default, however a BGP import policy can be used to apply a non-default damping profile to the route, and the half-life in the non-default damping profile can have any value between 1 and 45 minutes.

• The FOM value at the last time of update can be displayed using the show router bgp damping detail command. The time of last update can be up to 640 seconds ago; SR OS does not calculate the current FOM every time the show command is entered.

• When the FOM reaches the suppress limit, which is 3000 by default, but can be changed to any value between 1 and 20000 in a non-default damping profile, the route is suppressed, meaning it is not used locally and not advertised to peers. The route remains suppressed until either the FOM exponentially decays to a value less than or equal to the reuse threshold or the max-suppress time is reached. By default, the reuse threshold is 750 and the max-suppress time is 60 minutes, but these can be changed in a non-default damping profile: reuse can have a value between 1 and 20000 and max-suppress can have a value between 1 and 720 minutes.

The export command is used to apply one or more policies (up to 15) to a neighbor, group or to the entire BGP context. The export command that is most-specific to a peer is the one that is applied. An export policy command applied at the neighbor level takes precedence over the same command applied at the group or global level. An export policy command applied at the group level takes precedence over the same command specified on the global level. The export policies applied at different levels are not cumulative. The policies listed in an export command are evaluated in the order in which they are specified.

The most common uses for BGP export policies are as follows.

• To locally originate a BGP route by exporting (or redistributing) a non-BGP route that is installed in the route table and actively used for forwarding. The non-BGP route is most frequently a direct, static or aggregate route (exporting IGP routes into BGP is generally not recommended).

• To block the advertisement of specific BGP routes toward specific BGP peers. The routes may be blocked on the basis of IP prefix, communities, and so on.

• To modify the attributes of BGP routes advertised to specific BGP peers. The following path attribute modifications are possible using BGP export policies.

  • Change the ORIGIN value.

  • Add a sequence of AS numbers to the start of the AS_PATH. When a route is advertised to an EBGP peer the addition of the local-AS/global-AS numbers to the AS_PATH is always the final step (done after export policy).

  • Replace the AS_PATH with a new AS_PATH. When a route is advertised to an EBGP peer the addition of the local-AS/global-AS numbers to the AS_PATH is always the final step (done after export policy).

  • Prepend an AS number multiple times to the start of the AS_PATH. When a route is advertised to an EBGP peer the addition of the local-AS/global-AS numbers to the AS_PATH is always the final step (done after export policy). The add/replace action on the AS_PATH supersedes the prepend action if both are specified in the same policy entry.

  • Change the NEXT_HOP to a specific IP address. When a route is advertised to an EBGP peer the next-hop cannot be changed from the local-address.

  • Change the NEXT_HOP to the local-address used with the peer (next-hop-self).

  • Add a value to the MED. If the MED attribute does not exist it is added.

  • Subtract a value from the MED. If the MED attribute does not exist it is added with a value of 0. If the result of the subtraction is a negative number the MED metric is set to 0.

  • Set the MED to a particular value.

  • Set the MED to the cost of the IP route (or tunnel) used to resolve the BGP next-hop.

  • Set LOCAL_PREF to a particular value when advertising to an IBGP peer.

  • Add, remove or replace standard communities.

  • Add, remove or replace extended communities.

  • Add a static value to the AIGP metric when advertising the route to an AIGP-enabled peer with a modified BGP next-hop. The static value is incremental to the automatic adjustment of the LOC-RIB AIGP metric to reflect the distance between the local router and the received BGP next-hop.

  • Increment the AIGP metric by a fixed amount when advertising the route to an AIGP-enabled peer with a modified BGP next-hop. The static value is a substitute for the dynamic value of the distance between the local router and the received BGP next-hop.

Outbound Route Filtering (ORF) is a mechanism that allows one router, the ORF-sending router to signal to a peer, the ORF-receiving router, a set of route filtering rules (ORF entries) that the ORF-receiving router should apply to its route advertisements toward the ORF-sending router. The ORF entries are encoded in Route Refresh messages.

The use of ORF on a session must be negotiated — that is, both routers must advertise the ORF capability in their Open messages. The ORF capability describes the address families that support ORF, and for each address family, the ORF types that are supported and the ability to send/receive each type. 7450, 7750, and 7950 routers support ORF type 3, which is ORF based on Extended Communities. It is supported for only the following address families:

• VPN-IPv4

• VPN-IPv6

• MVPN-IPv4

• MVPN-IPv6

In SR OS the send/receive capability for ORF type 3 is configurable (with the send-orf and accept-orf commands) but the setting applies to all supported address families.

SR OS support for ORF type 3 allows a PE router that imports VPN routes with a particular set of Route Target Extended Communities to indicate to a peer (for example a route reflector) that it only wants to receive VPN routes that contain one or more of these Extended Communities. When the PE router wants to inform its peer about a new RT Extended Community it sends a Route Refresh message to the peer containing an ORF type 3 entry instructing the peer to add a permit entry for the 8-byte extended community value. When the PE router wants to inform its peer about a RT Extended Community that is no longer needed it sends a Route Refresh message to the peer containing an ORF type 3 entry instructing the peer to remove the permit entry for the 8-byte extended community value.

In SR OS the type-3 ORF entries that are sent to a peer can be generated dynamically (if no Route Target Extended Communities are specified with the send-orf command) or else specified statically. Dynamically generated ORF entries are based on the route targets that are imported by all locally-configured VPRNs.

A router that has installed ORF entries received from a peer can still apply BGP export policies to the session. If the evaluation of a BGP export policy results in a reject action for a VPN route that matches a permit ORF entry the route is not advertised (that is, the export policy has the final word).

Despite the advantages of ORF compared to manually configured BGP export policies a better technology, when it comes to dynamic filtering based on Route Target Extended Communities, is RT Constraint. RT Constraint is discussed further in the next section.

RT constrained route distribution, or RT-constrain for short, is a mechanism that allows a router to advertise to specific peers a special type of MP-BGP route called an RTC route; the associated AFI is 1 and the SAFI is 132. The NLRI of an RTC route encodes an Origin AS and a Route Target Extended Community with prefix-type encoding (for instance, if there is a prefix-length and ‟host” bits after the prefix-length are set to zero). A peer receiving RTC routes does not advertise VPN routes to the RTC-sending router unless they contain a Route Target Extended Community that matches one of the received RTC routes. As with any other type of BGP route RTC routes are propagated loop-free throughout and between Autonomous Systems. If there are multiple RTC routes for the same NLRI the BGP decision process selects one as the best path. The propagation of the best path installs RIB-OUT filter rules as it travels from one router to the next and this process creates an optimal VPN route distribution tree rooted at the source of the RTC route.

In SR OS the capability to exchange RTC routes is advertised when the route-target keyword is added to the relevant family command. RT-constrain is supported on EBGP and IBGP sessions of the base router instance. On any particular session either ORF or RT-constrain may be used but not both; if RT-constrain is configured the ORF capability is not announced to the peer.

When RT-constrain has been negotiated with one or more peers SR OS automatically originates and advertises to these peers one /96 RTC route (the origin AS and Route Target Extended Community are fully specified) for every route target imported by a locally-configured VPRN or BGP-based L2 VPN; this includes MVPN-specific route targets.

SR OS also supports a group/neighbor level default-route-target command that causes routers to generate and send a 0:0:0/0 default RTC route to one or more peers. Sending the default RTC route to a peer conveys a request to receive all VPN routes from that peer. The default-route-target command is typically configured on sessions that a route reflector has with its PE clients. A received default RTC route is never propagated to other routers.

The advertisement of RTC routes by a route reflector follows special rules that are described in RFC 4684. These rules are needed to ensure that RTC routes for the same NLRI that are originated by different PE routers in the same Autonomous System are properly distributed within the AS.

When a BGP session comes up, and RT-constrain is enabled on the session (both peers advertised the MP-BGP capability), routers delay sending any VPN-IPv4 and VPN-IPv6 routes until either the session has been up for 60 seconds or the End-of-RIB marker is received for the RT-constrain address family. When the VPN-IPv4 and VPN-IPv6 routes are sent they are filtered to include only those with a Route Target Extended Community that matches an RTC route from the peer. VPN-IP routes matching an RTC route originated in the local AS are advertised to any IBGP peer that advertises a valid path for the RTC NLRI. In other words, route distribution is not constrained to only the IBGP peer advertising the best path. On the other hand, VPN-IP routes matching an RTC route originated outside the local AS are only advertised to the EBGP or IBGP peer that advertises the best path.

According to the BGP standard (RFC 4271), a BGP router should not send updated reachability information for an NLRI to a BGP peer until a specific period of time, Min Route Advertisement Interval (MRAI), has elapsed from the last update. The RFC suggests the MRAI should be configurable per peer but does not propose a specific algorithm, and therefore, MRAI implementation details vary from one router operating system to another.

In SR OS, the MRAI is configurable, on a per-session basis, using the min-route-advertisement command. The min-route-advertisement command can be configured with any value between 1 and 255 seconds and the setting applies to all address families. The default value is 30 seconds, regardless of the session type (EBGP or IBGP). The MRAI timer is started at the configured value when the session is established and counts down continuously, resetting to the configured value whenever it reaches zero. Every time it reaches zero, all pending RIB-OUT routes are sent to the peer.

To send UPDATE messages that advertise new NLRI reachability information more frequently for some address families than others, SR OS offers a rapid-update command that overrides the remaining time on a peer's MRAI timer and immediately sends routes belonging to specified address families (and all other pending updates) to the peers receiving these routes. The address families that can be configured with rapid-update support are:

• EVPN

• L2-VPN

• label-IPv4

• label-IPv6

• MCAST-VPN-IPv4

• MCAST-VPN-IPv6

• MDT-SAFI

• MVPN-IPv4

• MVPN-IPv6

• VPN-IPv4

• VPN-IPv6

In many cases, the default MRAI is appropriate for all address families (or at least those not included in the preceding list) when it applies to UPDATE messages that advertise reachable NLRI, but it is not the best option for UPDATE messages that advertise unreachable NLRI (route withdrawals). Fast re-convergence after some types of failures requires route withdrawals to propagate to other routers as quickly as possible so that they can calculate and start using new best paths, which would be impeded by the effect of the MRAI timer at each router hop. This is facilitated by the rapid-withdrawal configuration command.

When rapid-withdrawal is configured, UPDATE messages containing withdrawn NLRI are sent immediately to a peer without waiting for the MRAI timer to expire. UPDATE messages containing reachable NLRI continue to wait for the MRAI timer to expire, or for a rapid-update trigger, if it applies. When rapid-withdrawal is enabled, it applies to all address families.

When there is a change to a labeled-unicast route that requires reprogramming of the label operations in the dataplane, these IOM updates are not made until the changed route is advertised to a peer, which depends on MRAI. Lowering the MRAI value or using rapid-update improves the speed of this operation.

BGP does not allow a route to be advertised unless it is the best path in the RIB and an export policy allows the advertisement.

In some cases, it may be useful to advertise the best BGP path to peers despite the fact that is inactive. For example, because there are one or more preferred non-BGP routes to the same destination and one of these other routes is the active route. One way SR OS supports this flexibility is using the advertise-inactive command; other methods include best-external and add-paths.

When the BGP advertise-inactive command is configured so that it applies to a BGP session it has the following effect on the IPv4, IPv6, mcast-ipv4, mcast-ipv6, label-IPv4 and label-IPv6 routes advertised to that peer.

• If the active route for the IP prefix is a BGP route then that route is advertised. If the active route for the IP prefix is a non-BGP route and there is at least one valid but inactive BGP route for the same destination then the best of the inactive and valid BGP routes is advertised unless the non-BGP active route is matched and accepted by an export policy applied to the session.

• If the active route for the IP prefix is a non-BGP route and there are no (valid) BGP routes for the same destination then no route is advertised for the prefix unless the non-BGP active route is matched and accepted by an export policy applied to the session.

Best-external is a BGP enhancement that allows a BGP speaker to advertise to its IBGP peers its best ‟external” route for a prefix/NLRI when its best overall route for the prefix/NLRI is an ‟internal” route. This is not possible in a normal BGP configuration because the base BGP specification prevents a BGP speaker from advertising a non-best route for a destination.

In specific topologies best-external can improve convergence times, reduce route oscillation and allow better loadsharing. This is achieved because routers internal to the AS have knowledge of more exit paths from the AS. Enabling add-paths on border routers of the AS can achieve a similar result but add-paths introduces NLRI format changes that must be supported by BGP peers of the border router and therefore has more interoperability constraints than best-external (which requires no messaging changes).

Best-external is supported in the base router BGP context. (A related feature is also supported in VPRNs; consult the Services Guide for more details.) It is configured using the advertise-external command, which provides IPv4, label-IPv4, IPv6, and label-IPv6 as options.

The advertisement rules when advertise-external is enabled can be summarized as follows.

• If a router has advertise-external enabled and its best overall route is a route from an IBGP peer then this best route is advertised to EBGP and confed-EBGP peers, and the ‟best external” route is advertised to IBGP peers. The ‟best external” route is the one found by running the BGP path selection algorithm on all LOC-RIB paths except for those learned from the IBGP peers.

  Note: A route reflector with advertise-external enabled does not include IBGP routes learned from other clusters in its definition of ‛external’.

• If a router has advertise-external enabled and its best overall route is a route from an EBGP peer then this best route is advertised to EBGP, confed-EBGP, and IBGP peers.

• If a router has advertise-external enabled and its best overall route is a route from a confed-EBGP peer in member AS X then this best route is advertised to EBGP, IBGP peers and confed-EBGP peers in all member AS except X and the ‟best external” route is advertised to confed-EBGP peers in member AS X. In this case the ‟best external” route is the one found by running the BGP path selection algorithm on all RIB-IN paths except for those learned from member AS X.

  Note: If the best-external route is not the best overall route it is not installed in the forwarding table and in some cases this can lead to a short-duration traffic loop after failure of the overall best path.

Add-paths is a BGP enhancement that allows a BGP router to advertise multiple distinct paths for the same prefix/NLRI. Add-Paths provides a number of potential benefits, including reduced routing churn, faster convergence, and better loadsharing.

For a router to receive multiple paths per NLRI from a peer, for a particular address family, the peer must announce the BGP capability to send multiple paths for the address family and the local router must announce the BGP capability to receive multiple paths for the address family. When the Add-Path capability has been negotiated this way, all advertisements and withdrawals of NLRI by the peer must include a path identifier. The path identifier has no significance to the receiving router. If the combination of NLRI and path identifier in an advertisement from a peer is unique (does not match an existing route in the RIB-IN from that peer) then the route is added to the RIB-IN. If the combination of NLRI and path identifier in a received advertisement is the same as an existing route in the RIB-IN from the peer then the new route replaces the existing one. If the combination of NLRI and path identifier in a received withdrawal matches an existing route in the RIB-IN from the peer, then that route is removed from the RIB-IN.

An UPDATE message carrying an IPv4 NLRI with a path identifier is shown in BGP update message with path identifier for IPv4 NLRI.

Add-paths is only supported by the base router BGP instance and the EBGP and IBGP sessions it forms with other peers capable of add-paths. The ability to send and receive multiple paths per prefix is configurable per family, and supported for the following options:

• IPv4

• label-IPv4

• VPN-IPv4

• IPv6

• label-IPv6

• VPN-IPv6

• MCAST-VPN-IPv4

• MCAST-VPN-IPv6

• MVPN-IPv4

• MVPN-IPv6

• EVPN

Split-horizon refers to the action taken by a router to avoid advertising a route back to the peer from which it was received. By default, SR OS applies split-horizon behavior only to routes received from IBGP non-client peers, and split-horizon only works for routes to non-imported routes within a RIB. This split-horizon functionality, which can never be disabled, prevents a route learned from a non-client IBGP peer to be advertised to the sending peer or any other non-client peer.

To apply split-horizon behavior to routes learned from RR clients, confed-EBGP peers or (non-confed) EBGP peers the split-horizon command must be configured in the appropriate contexts; it is supported at the global BGP, group and neighbor levels. When split-horizon is enabled on these types of sessions, it only prevents the advertisement of a route back to its originating peer; for example, SR OS does not prevent the advertisement of a route learned from one EBGP peer back to a different EBGP peer in the same neighbor AS.

Received non-VPN-aware FlowSpec routes are validated following the procedures described in RFC 5575 and draft-ietf-idr-bgp-flowspec-oid-03, Revised Validation Procedure for BGP Flow Specifications. Configure the validate-dest-prefix command in a routing instance to enable validation checks based on the destination prefix. By default, this check is not performed. When the command is enabled, BGP determines whether a non-VPN-aware FlowSpec route is valid or invalid based on the following logic:

1. If the FlowSpec route was originated in the same Autonomous System (AS) as the receiving BGP router, it is automatically valid.

2. If rule 1 does not apply, the FlowSpec route was originated in an external AS, and it does not contain a destination prefix subcomponent, it is considered valid.

3. If rule 1 does not apply, the FlowSpec route was originated in an external AS, and it does contain a destination prefix subcomponent, it is considered valid if all of the following are true:

   • The neighbor AS (last non-confed AS in the AS_PATH) of the FlowSpec route matches the neighbor AS of the unicast IP route that is the best match of the destination prefix. The best match unicast IP route must be a BGP route (that is, not static, IGP, or other routes).

   • The neighbor AS of the FlowSpec route matches the neighbor AS of all unicast IP routes that are longer matches of the destination prefix. All longer match unicast IP routes must be BGP routes (that is, not static, IGP, or other routes).

Non-VPN-aware FlowSpec routes that are received with a redirect-to-IPv4 extended community action are also subject to an additional set of validation checks. If the validate-redirect-ip command is enabled in the receiving BGP instance, a non-VPN-aware FlowSpec route is considered invalid, if it is deemed to have originated in a different AS than the IP route that resolves the redirection IPv4 address. The originating AS of a FlowSpec route is determined from its AS paths.

A FlowSpec route that is determined to be invalid by any of the validation rules described earlier is retained in the BGP RIB, but not used for traffic filtering and not propagated to other BGP speakers.

When the base router BGP instance receives a non-VPN-aware flow IPv4 or IPv6 route that is considered valid and best, the system attempts to construct an IPv4 or IPv6 filter entry from the NLRI contents and actions encoded in the UPDATE message. If successful, the filter entry is added to the system-created ‟fSpec-0” IPv4 embedded filter or to the ‟fSpec-0” IPv6 embedded filter. These embedded filters can be inserted into configured IPv4 and IPv6 filter policies that are applied to ingress traffic on a selected set of the base router IP interfaces. These interfaces can include network interfaces, IES SAP interfaces, and IES spoke SDP interfaces.

When the VPRN BGP instance receives a non-VPN-aware flow IPv4 or IPv6 route from a BGP peer of the VPRN or imports a VPN-aware FlowSpec-VPN IPv4 or IPv6 route that was received in the base router BGP instance and considered to be valid and best, the system attempts to construct an IPv4 or IPv6 filter entry from the NLRI contents and actions encoded in the UPDATE message. If successful, the filter entry is added to the system-created "fSpec-$vprnid" IPv4 embedded filter or to the "fSpec-$vprnid" IPv6 embedded filter. $vprnid represents a parameter value that is unique to each VPRN. These embedded filters can be inserted into configured IPv4 and IPv6 filter policies that are applied to ingress traffic on one or more of the IP interfaces on the VPRN.

When FlowSpec rules are embedded into a user-defined filter policy, configure the insertion point of the rules using the following commands:

• MD-CLI

  configure filter ip-filter embed flowspec offset
  configure filter ipv6-filter embed flowspec offset

• classic CLI

  configure filter ip-filter embed-filter flowspec offset
  configure filter ipv6-filter embed-filter flowspec offset

The sum of the configured maximum number of FlowSpec routes and offset must not exceed the maximum filter entry ID range.

For IPv4 and IPv6 packets forwarded using a RFC 8277 label route in the global routing instance, including label-IPv6, the following command specified with the all value enables TTL propagation from the IP header into all labels in the transport label stack:

• config router ttl-propagate label-route-local [none | all]

• config router ttl-propagate label-route-transit [none | all]

The none value reverts to the default mode which disables TTL propagation from the IP header to the labels in the transport label stack.

These commands do not have a no version.

• RSVP LSP shortcut:

  • configure router mpls shortcut-transit-ttl-propagate

  • configure router mpls shortcut-local-ttl-propagate

• LDP LSP shortcut:

  • configure router ldp shortcut-transit-ttl-propagate

  • configure router ldp shortcut-local-ttl-propagate

This feature does not impact packets forwarded over BGP shortcuts. The ingress LER operates in uniform mode by default and can be changed into pipe mode using the configuration of TTL propagation for RSVP or LDP LSP shortcut.

This feature configures the TTL propagation for transit packets at a router acting as an LSR for a BGP label route.

When an LSR swaps the BGP label for a IPv4 prefix packet, therefore acting as a ABR, ASBR, or data-path Route-Reflector (RR) in the base routing instance, or swaps the BGP label for a vpn-IPv4 or vpn-IPv6 prefix packet, therefore acting as an inter-AS Option B VPRN ASBR or VPRN data path Route-Reflector (RR), the all value of the following command enables TTL propagation of the decremented TTL of the swapped BGP label into all LDP or RSVP transport labels.

configure router ttl-propagate lsr-label-route [none | all]

When an LSR swaps a label or stitches a label, it always writes the decremented TTL value into the outgoing swapped or stitched label. What the above CLI controls is whether this decremented TTL value is also propagated to the transport label stack pushed on top of the swapped or stitched label.

The none value reverts to the default mode which disables TTL propagation. This changes the existing default behavior which propagates the TTL to the transport label stack. When a customer upgrades, the new default becomes in effect. The above commands do not have a no version.

The following describes the behavior of LSR TTL propagation in a number of other use cases and indicates if the above CLI command applies or not.

• When an LSR stitches an LDP label to a BGP label, the decremented TTL of the stitched label is propagated or not to the LDP or RSVP transport labels as per the above configuration.

• When an LSR stitches a BGP label to an LDP label, the decremented TTL of the stitched label is automatically propagated into the RSVP label if the outgoing LDP LSP is tunneled over RSVP. This behavior is not controlled by the above CLI.

• When an LSR pops a BGP label and forwards the packet using an IGP route (IGP route to destination of prefix wins over the BGP label route), it pushes an LDP label on the packet and the TTL behavior is as described in the previous bullet when stitching from a BGP label to an LDP label.

• When an ingress Carrier Supporting Carrier (CsC) PE swaps the incoming EBGP label into a VPN-IPv4 label. The reverse operation is performed by the egress CsC PE. In both cases, the decremented TTL of the swapped label is or is not passed on to the LDP or RSVP transport labels as per the above configuration.

The following BGP-LS components are currently supported.

Protocol-ID:

• IS-IS level 1

• IS-IS level 2

• OSPFv2

NLRI Types:

• Node NLRI

• Link NLRI

• IPv4 Topology Prefix NLRI

Node Descriptor TLVs:

• 512 — Autonomous System

• 513 — BGP-LS Identifier

• 514 — OSPF Area-ID

• 515 — IGP Router-ID

Node Attribute TLVs:

• 1024 — Node Flag Bits (O and B bits supported)

• 1028 — IPv4 Router-ID of Local Node (only supported for IS-IS)

• Segment Routing:

  • 1034 — SR Capabilities (only supported for IS-IS)

  • 1035 — SR Algorithm (only supported for IS-IS)

Link Descriptors TLVs:

• 258 — Link Local/Remote Identifiers

• 259 — IPv4 interface address

• 260 — IPv4 neighbor address

Link Attributes TLVs:

• 1028 — IPv4 Router-ID of Local Node (only supported for IS-IS)

• 1088 — Administrative group (color)

• 1089 — Maximum link bandwidth

• 1090 — Max. reservable link bandwidth

• 1091 — Unreserved bandwidth

• 1092 — TE Default Metric

• 1095 — IGP Metric

• 1096 — Shared Risk Link Group

• Segment Routing:

  • 1099 — Adjacency Segment Identifier

  • 1100 — LAN Adjacency Segment Identifier

Prefix Descriptors TLVs:

• 264 — OSPF Route Type (only Intra-Area and Inter-Area)

• 265 — IP Reachability Information

Prefix Attributes TLVs:

• 1152 — IGP Flags (only D flag supported)

• 1155 — Prefix Metric

• Segment Routing:

  • 1158 — Prefix SID

  • 1159 — Range (prefix-SID and sub-TLV only)

  • 1170 — IGP Prefix Attributes (only supported for OSPF)

In addition to enabling the BGP-LS route family for a BGP neighbor, the following CLI is required to send the Egress Peering Segments described in BGP Egress Peer Engineering using BGP Link State using the NLRI Type 2 with protocol ID set to BGP-EPE.

CLI syntax

configure>router>bgp
   egress-peer-engineering {  
     admin-state {enable | disable}
   }

When egress-peer-engineering is administratively enabled, BGP registers with SR and the router starts advertising any peer node and peer adjacency SIDs in BGP-LS.

To allocate peer node and peer adjacency SIDs, use the following syntax to configure the egress-engineering command and enable BGP-EPE for a BGP neighbor or group.

CLI syntax

configure>router>bgp
  group
    neighbor <a.b.c.d> {
      egress-engineering {
        admin-state {enable | disable}
      }

configure>router>bgp
  group
    egress-engineering {
      admin-state {enable | disable}
    }

The BGP egress-engineering at the neighbor level overrides the group level configuration. When a neighbor does not have an egress-engineering configuration context, the group configuration is inherited in the following cases.

• If the group does not have an egress-engineering configuration, egress-engineering is disabled for the neighbor.

• If the group has an egress-engineering configuration in the default disabled state, egress-engineering is disabled for the neighbor.

• If the group has an enabled egress-engineering configuration, egress-engineering is enabled for the neighbor.

When a neighbor has egress-engineering configured and in the default disabled state, egress-engineering is disabled for the neighbor, irrespective of the disabled, enabled, or no-context configuration at the group level. When a neighbor has egress-engineering configured and enabled, egress-engineering is enabled for the neighbor, irrespective of the disabled, enabled, or no-context configuration at the group level.

By default, enabling egress-engineering at the peer or group level causes SID values (MPLS labels) to be dynamically allocated for the peer node segment and the peer adjacency segments. Although the labels are assigned when the neighbor or group is configured, they are not programmed until the adjacency comes up. Peer node segments are derived from the BGP next hops used to reach a specific peer. If the node reboots, these dynamically allocated label values may change and are re-announced in BGP-LS.

If a BGP neighbor goes down, the router advertises a delete for all SIDs associated with the neighbor and deprograms them from the IOM. However, the label values for the SIDs are not released and the router re-advertises the same values when the BGP neighbor comes back up.

If a BGP neighbor is deleted from the configuration or is shut down, or egress-engineering is disabled, the router advertises a delete for all SIDs associated with the neighbor and deprograms them from the IOM. The router also releases the label values for the SIDs.

The following list summarizes the BGP configuration defaults.

• By default, the router is not assigned to an AS.

• A BGP instance is created in the administratively enabled state.

• A BGP group is created in the administratively enabled state.

• A BGP neighbor is created in the administratively enabled state.

• No BGP router ID is specified. If no BGP router ID is specified, BGP uses the router system interface address.

• The router BGP timer defaults are generally the values recommended in IETF drafts and RFCs (see BGP MIB notes)

• If no import route policy statements are specified, then all BGP routes are accepted.

• If no export route policy statements specified, then all best and used BGP routes are advertised and non-BGP routes are not advertised.

The router implementation of the RFC 1657 MIB variables listed in SR OS and IETF MIB variations differs from the IETF MIB specification.

If SNMP is used to set a value of X to the MIB variable in SR OS and IETF MIB variations , there are three possible results:

When the value set using SNMP is within the IETF allowed values and outside the SR OS values as specified in SR OS and IETF MIB variations and MIB variable with SNMP , a log message is generated.

The log messages that display are similar to the following log messages:

Sample Log Message for setting bgpPeerMinRouteAdvertisementInterval to 256

535 2006/11/12 19:40:53 [Snmpd] BGP-4-bgpVariableRangeViolation: Trying to set 
bgpPeerMinRouteAdvInt to 256 - valid range is [2-255] - setting to 255

Sample Log Message for setting bgpPeerMinRouteAdvertisementInterval to 1

566 2006/11/12 19:44:41 [Snmpd] BGP-4-bgpVariableRangeViolation: Trying to set 
bgpPeerMinRouteAdvInt to 1 - valid range is [2-255] - setting to 2

Before BGP can be implemented, the following entities must be configured:

• The autonomous system (AS) number for the router

  An AS number is a globally unique value which associates a router to a specific autonomous system. This number is used to exchange exterior routing information with neighboring ASs and as an identifier of the AS itself. Each router participating in BGP must have an AS number specified.

  To implement BGP, the AS number must be specified in the config>router context.

• Router ID

  The router ID is the IP address of the local router. The router ID identifies a packet’s origin. The router ID must be a valid host address.

BGP is configured in the config>router>bgp context. Three hierarchical levels are included in BGP configurations:

• global level

• group level

• neighbor level

Commands and parameters configured on the global level are inherited to the group and neighbor levels although parameters configured on the group and neighbor levels take precedence over global configurations.

A BGP system comprises of ASs which share network reachability information. Network reachability information is shared with adjacent BGP peers. BGP supports two types of routing information exchanges.

• External BGP (EBGP) is used between ASs

  EBGP speakers peer to different ASs and typically share a subnet. In an external group, the next hop is dependent upon the interface shared between the external peer and the specific neighbor. The multihop command must be specified if an EBGP peer is more than one hop away from the local router.

• Internal BGP (IBGP) is used within an AS

  IBGP peers belong to the same AS and typically does not share a subnet. Neighbors do not have to be directly connected to each other. Because IBGP peers are not required to be directly connected, IBGP uses the IGP path (the IP next-hop learned from the IGP) to reach an IBGP peer for its peering connection.

A newly created or existing BGP instance, group, or EBGP neighbor in a classic interface (the classic CLI and SNMP) maintains backwards compatibility with the insecure default to advertise and receive all routes. It is not compliant with RFC 8212. The secure default behavior must be enabled using the ebgp-default-reject-policy command in these cases.

A newly created BGP instance, group, or EBGP neighbor in a model-driven interface (the MD-CLI, NETCONF, or gRPC) applies the secure default behavior to reject all routes. It is compliant with RFC 8212. The secure behavior can be disabled using the ebgp-default-reject-policy command. However, Nokia recommends configuring import and export policies that express the intended routing instead of using the insecure default behavior. Defining an empty policy does not match any routes, an accept must match the route through an action accept or default-action accept statement.

The default behavior is inherited from the BGP instance to the group to an EBGP neighbor.

The import and export policies that are applied can be displayed using info detail or the show router bgp neighbor commands.

Default EBGP route propagation behavior shows the default EBGP route propagation behavior according to how the neighbor was configured.