IS-IS

OSI IS-IS routing uses two-level hierarchical routing. A routing domain can be partitioned into areas. Level 1 routers know the topology in their area, including all routers and end systems in their area but do not know the identity of routers or destinations outside of their area. Level 1 routers forward traffic with destinations outside of their area to a level 2 router in their area.

Level 2 routers know the level 2 topology, and know which addresses are reachable by each level 2 router. Level 2 routers do not need to know the topology within any level 1 area, except to the extent that a level 2 router can also be a level 1 router within a single area. By default, only level 2 routers can exchange PDUs or routing information directly with external routers located outside the routing domain.

The two types of routers in IS-IS are described below.

• Level 1 intermediate systems

  Routing is performed based on the area ID portion of the ISO address called the Network Entity Title (NET). Level 1 systems route within an area. They recognize, based on the destination address, whether the destination is within the area. If so, they route toward the destination. If not, they route to the nearest level 2 router.

• Level 2 intermediate systems

  Routing is performed based on the area address. They route toward other areas, regardless of other area’s internal structure. A level 2 intermediate system can also be configured as a level 1 intermediate system in the same area.

The level 1 router’s area address portion is manually configured (see ISO network addressing). A level 1 router does not become a neighbor with a node that does not have a common area address. However, if a level 1 router has area addresses A, B, and C, and a neighbor has area addresses B and D, then the level 1 router accepts the other node as a neighbor, as address B is common to both routers. Level 2 adjacencies are formed with other level 2 nodes whose area addresses do not overlap. If the area addresses do not overlap, the link is considered by both routers to be level 2 only and only level 2 LSPDUs flow on the link.

Within an area, level 1 routers exchange LSPs which identify the IP addresses reachable by each router. Specifically, zero or more IP address, subnet mask, and metric combinations can be included in each LSP. Each level 1 router is manually configured with the IP address, subnet mask, and metric combinations, which are reachable on each interface. A level 1 router routes as follows:

• if a specified destination address matches an IP address, subnet mask, or metric reachable within the area; the PDU is routed via level 1 routing

• if a specified destination address does not match any IP address, subnet mask, or metric combinations listed as reachable within the area; the PDU is routed toward the nearest level 2 router

Level 2 routers include in their LSPs, a complete list of IP address, subnet mask, and metrics specifying all the IP addresses which reachable in their area. This information can be obtained from a combination of the level 1 LSPs (by level 1 routers in the same area). Level 2 routers can also report external reachability information, corresponding to addresses reachable by routers in other routing domains or autonomous systems.

• Area

  An area is a routing sub-domain which maintains detailed routing information about its own internal composition, and also maintains routing information which allows it to reach other routing sub-domains. Areas correspond to the level 1 sub-domain.

• End system

  End systems send NPDUs to other systems and receive NPDUs from other systems, but do not relay NPDUs. This International Standard does not specify any additional end system functions beyond those supplied by ISO 8473 and ISO 9542.

• Neighbor

  A neighbor is an adjacent system reachable by traversing a single sub-network by a PDU.

• Adjacency

  An adjacency is a portion of the local routing information which pertains to the reachability of a single neighboring end or intermediate system over a single circuit. Adjacencies are used as input to the decision process to form paths through the routing domain. A separate adjacency is created for each neighbor on a circuit and for each level of routing (level 1 and level 2) on a broadcast circuit.

• Circuit

  The subset of the local routing information base pertinent to a single local Subnetwork Point of Attachments (SNPAs).

• Link

  The communication path between two neighbors. A link is up when communication is possible between the two SNPAs.

• Designated IS

  The intermediate system on a LAN which is designated to perform additional duties. In particular, the designated IS generates link-state PDUs on behalf of the LAN, treating the LAN as a pseudonode.

• Pseudonode

  Where a broadcast sub-network has n connected intermediate systems, the broadcast sub-network itself is considered to be a pseudonode. The pseudonode has links to each of the n intermediate systems and each of the ISs has a single link to the pseudonode (instead of n-1 links to each of the other intermediate systems). Link-state PDUs are generated on behalf of the pseudonode by the designated IS.

• Broadcast sub-network

  A multi-access subnetwork that supports the capability of addressing a group of attached systems with a single PDU.

• General topology sub-network

  A topology that is modeled as a set of point-to-point links, each of which connects two systems. There are several generic types of general topology subnetworks, multipoint links, permanent point-to-point links, dynamic and static point-to-point links.

• Routing sub-domain

  A routing sub-domain consists of a set of intermediate systems and end systems located within the same routing domain.

• Level 2 sub-domain

  Level 2 sub-domain is the set of all level 2 intermediate systems in a routing domain.

IS-IS uses ISO network addresses. Each address identifies a point of connection to the network, such as a router interface, and is called a Network Service Access Point (NSAP).

An end system can have multiple NSAP addresses, in which case the addresses differ only by the last byte (called the n selector). Each NSAP represents a service that is available at that node. In addition to having multiple services, a single node can belong to multiple areas.

Each network entity has a special network address called a Network Entity Title (NET). Structurally, a NET is identical to an NSAP address but has an n-selector of 00. Most end systems have one NET. Intermediate systems can have up to three area IDs (area addresses).

NSAP addresses are divided into three parts. Only the area ID portion is configurable.

• Area ID

  A variable length field between 1 and 13 bytes long. This includes the Authority and Format Identifier (AFI) as the most significant byte and the area ID.

• System ID

  A six-byte system identification. This value is not configurable. The system ID is derived from the system or router ID.

• Selector ID

  A one-byte selector identification that must contain zeros when configuring a NET. This value is not configurable. The selector ID is always 00.

Of the total 20 bytes comprising the NET, only the first 13 bytes, the area ID portion, can be manually configured. As few as one byte can be entered or, at most, 13 bytes. If less than 13 bytes are entered, the rest is padded with zeros.

Routers with common area addresses form level 1 adjacencies. Routers with no common NET addresses form level 2 adjacencies, if they are capable (Using area addresses to form adjacencies).

IS-IS route summarization allows users to create aggregate IPv4 or IPv6 addresses that include multiple groups of IPv4 or IPv6 addresses for a specific IS-IS level. IPv4 and IPv6 routes redistributed from other routing protocols also can be summarized. This is similar to OSPF configuration using the area-range command. IS-IS IPv4 and IPv6 route summarization helps to reduce the size of the LSDB and the IPv4 or IPv6 routing table, and reduces the chance of route flapping.

IS-IS route summarization supports:

• level 1, level 1-2, and level 2

• route summarization for the IPv4 or IPv6 routes redistributed from other protocols

• the smallest metric used to advertise summary addresses of all the more specific IPv4 or IPv6 routes

• IS-IS IPv6 route summarization algorithm and SRv6 locator awareness

Multi-Topology IS-IS (MT-ISIS) support within SR OS allows for the creation of different topologies within IS-IS that contribute routes to specific route tables for IPv4 unicast, IPv6 unicast, IPv4 multicast, and IPv6 multicast. This capability allows for non-congruent topologies between these different routing tables. As a result, networks are able to control which links or nodes are to be used for forwarding different types of traffic.

For example, MT-ISIS could allow all links to carry IPv4 traffic, while only a subset of links can also carry IPv6 traffic.

SR OS supports the following multi-topologies:

• IPv4 Unicast – MT-ID 0

• IPv6 Unicast – MT-ID 2

• IPv4 Multicast – MT-ID 3

• IPv6 Multicast – MT-ID 4

IS-IS administrative tags enable a network administrator to configure route tags to tag IS-IS route prefixes. These tags can subsequently be used to control IS-IS route redistribution or route leaking.

IS-IS route tagging can be applied to IP addresses of an interface and to administrative policies with a route map. A network administrator can tag a summary route and then use a route policy to match the tag with one or more attributes for the route.

Using these administrative policies, the operator can control how a router handles route exchanges with its IS-IS neighboring routers. Administrative policies are also used to govern the installation of routes in the routing table.

Route tags allow policies to do the following:

• redistribute routes received from other protocols in the routing table to IS-IS

• redistribute routes or SRv6 locators between levels in an IS-IS routing hierarchy

• summarize routes redistributed into IS-IS or within IS-IS by creating aggregate (summary) addresses

If the SR OS router is providing a grace period to an adjacent neighbor and the BFD session associated with that neighbor fails, the behavior is determined by the C-bit values sent by each neighbor.

• If both BFD end-points have set their C-bit value, then the graceful restart helper mode is canceled and any routes from that neighbor that are marked as stale are removed from the forwarding table.

• If either of the BFD end-points has not set their C-bit value, then the graceful restart helper mode continues.

• IS-IS must be enabled on each participating router.

• There are no default network entity titles.

• There are no default interfaces.

• By default, the routers are assigned a level 1/level 2 level capability.

For IS-IS to operate on the routers, IS-IS must be explicitly enabled, and at least one area address and interface must be configured. If IS-IS is enabled but no area address or interface is defined, the protocol is enabled but no routes are exchanged. When at least one area address and interface are configured, then adjacencies can be formed and routes exchanged.

The following output displays IS-IS default values.

*A:Dut-D>config>router>isis# info detail
----------------------------------------------
           no system-id
           no router-id
           level-capability level-1/2
           no graceful-restart
           no auth-keychain
           no authentication-key
           no authentication-type
           authentication-check
           csnp-authentication
           no ignore-lsp-errors
           no ignore-narrow-metric
           lsp-lifetime 1200
           lsp-mtu-size 1492
           lsp-refresh-interval 600
           no export-limit
           no export
           no import
           hello-authentication
           psnp-authentication
           no traffic-engineering
           no reference-bandwidth   
           no default-route-tag
           no disable-ldp-sync
           no advertise-passive-only
           no advertise-router-capability
           no hello-padding
           no ldp-over-rsvp
           no advertise-tunnel-link
           no ignore-attached-bit
           no suppress-attached-bit
           no iid-tlv-enable
           no poi-tlv-enable
           no prefix-limit
           no loopfree-alternates
           no rib-priority high
           ipv4-routing
           no ipv6-routing
           ipv4-multicast-routing native
           ipv6-multicast-routing native
           no multi-topology
           no unicast-import-disable both
           no multicast-import both
           no strict-adjacency-check
           igp-shortcut
               shutdown
               tunnel-next-hop
                   family ipv4
                       resolution disabled
                       resolution-filter
                           no rsvp
                           no sr-te
                   exit
                   family ipv6
                       resolution disabled
                       resolution-filter
                           no rsvp
                           no sr-te
                   exit
                   family srv4
                       resolution disabled
                       resolution-filter
                           no rsvp
                           no sr-te
                   exit
                   family srv6
                       resolution disabled
                       resolution-filter
                           no rsvp
                           no sr-te
                   exit
               exit
           exit
           timers
               lsp-wait 5000 lsp-initial-wait 10 lsp-second-wait 1000
               sfp-wait 10000 sfp-initial-wait 1000 sfp-second-wait 1000
           exit
           level 1
               advertise-router-capability
               no hello-padding
               no lsp-mtu-size
               no auth-keychain
               no authentication-key
               no authentication-type
               csnp-authentication
               external-preference 160
               hello-authentication
               no loopfree-alternate-exclude
               preference 15
               psnp-authentication
               no wide-metrics-only
               default-metric 10
              default-ipv4-multicast-metric 10
               default-ipv6-unicast-metric 10
               default-ipv6-multicast-metric 10
           exit
           level 2
               advertise-router-capability
               no hello-padding
               no lsp-mtu-size
               no auth-keychain
               no authentication-key
               no authentication-type
               csnp-authentication
               external-preference 165
              hello-authentication
               no loopfree-alternate-exclude
               preference 18
               psnp-authentication
               no wide-metrics-only
               default-metric 10
               default-ipv4-multicast-metric 10
               default-ipv6-unicast-metric 10
               default-ipv6-multicast-metric 10
           exit                     
           segment-routing
               shutdown
               adj-sid-hold 15
               no export-tunnel-table
               no prefix-sid-range
               no tunnel-table-pref
               no tunnel-mtu
               mapping-server
                   shutdown
               exit
           exit
           no shutdown

To implement IS-IS in your network, you must enable IS-IS on each participating router.

To assign different level to the routers and organize your network into areas, modify the level capability defaults on end systems from level 1/2 to level 1. Routers communicating to other areas can retain the level 1/2 default.

Use the CLI syntax displayed in the following subsections to configure IS-IS components.

The shutdown command disables the IS-IS protocol instance on the router. The configuration settings are not changed, reset, or removed.

To disable IS-IS on a router, enter the following commands:

config>router# isis
    shutdown

The no isis command deletes the IS-IS protocol instance. The IS-IS configuration reverts to the default settings.

To remove the IS-IS configuration enter the following commands:

config>router# 
    no isis

You can modify, disable, or remove global IS-IS parameters without shutting down entities. Changes take effect immediately. Modifying the level capability on the global level causes the IS-IS protocol to restart.

The following example displays command usage to modify various parameters.

Example

config>router>isis# overload timeout 500
config>router>isis# level-capability level-1/2
config>router>isis# no authentication-check
config>router>isis# authentication-key secretkey

The following example displays the global modifications.

A:ALA-A>config>router>isis# info
----------------------------------------------
        area-id 49.0180.0001
        area-id 49.0180.0002
        area-id 49.0180.0003
        authentication-key "//oZrvtvFPn06S42lRIJsE" hash
        authentication-type password
        no authentication-check
        overload timeout 500 on-boot
        level 1
            wide-metrics-only
        exit
        level 2
            wide-metrics-only
        exit
        interface "system"
        exit
        interface "ALA-1-2"
            level-capability level-2
        exit
        interface "ALA-1-3"
            level-capability level-1
            interface-type point-to-point
        exit
        interface "ALA-1-5"
            level-capability level-1
            interface-type point-to-point
        exit
        interface "to-103"
        exit
        interface "A-B"
        exit
        interface "A-C"
        exit
----------------------------------------------
A:ALA-A>config>router>isis#

You can modify, disable, or remove interface-level IS-IS parameters without shutting down entities. Changes take effect immediately. Modifying the level capability on the interface causes the IS-IS protocol on the interface to restart.

To remove an interface, issue the no interface ip-int-name command. To disable an interface, issue the shutdown command in the interface context.

The following example displays interface IS-IS modification command usage. For specific interface configuration and modification examples also see, Configuring a level 1 area and Modifying a router’s level capability.

Example

config>router# isis
config>router>isis# interface ALA-1-3
config>router>isis>if# passive
config>router>isis>if# exit
config>router>isis# interface to-103
config>router>isis>if# hello-authentication-type message-digest
config>router>isis>if# hello-authentication-key secretkey
config>router>isis>if# exit

The following example displays the modified interface parameters.

A:ALA-A>config>router>isis# info
----------------------------------------------
        area-id 49.0180.0001
        area-id 49.0180.0002
        area-id 49.0180.0003
        authentication-key "//oZrvtvFPn06S42lRIJsE" hash
        authentication-type password
        no authentication-check
        overload timeout 500 on-boot
        level 1
            wide-metrics-only
        exit
        level 2
            wide-metrics-only
        exit
        interface "system"
        exit
        interface "ALA-1-2"
            level-capability level-2
        exit
        interface "ALA-1-3"
            level-capability level-1
            interface-type point-to-point
            passive
        exit
        interface "ALA-1-5"
            level-capability level-1
            interface-type point-to-point
        exit
        interface "to-103"
            hello-authentication-key "DvR3l264KQ6vXMTvbAZ1mE" hash
            hello-authentication-type message-digest
        exit
        interface "A-B"
        exit
----------------------------------------------
A:ALA-A>config>router>isis#

The use of authentication mechanism is recommended to protect against malicious attack on the communications between routing protocol neighbors. These attacks could aim to either disrupt communications or to inject incorrect routing information into the systems routing table. The use of authentication keys can help to protect the routing protocols from these types of attacks. In addition, the use of authentication keychains provides the ability to configure authentication keys and make changes to them without affecting the state of the routing protocol adjacencies.

To configure the use of an authentication keychain within IS-IS, use the following steps.

1. Configure an authentication keychain within the config>system>security context. The configured keychain must include at least on valid key entry, using a valid authentication algorithm for the IS-IS protocol.

2. Associate the configure authentication keychain with IS-IS. Authentication keychains can be used to specify the authentication at the IS-IS global, and level context as well as for hello authentication at the interface and interface-level context.

The association of the authentication keychain is established through the auth-keychain keychain-name command at the global and level context. The hello authentication association is established through the hello-auth-keychain keychain-name command.

For a key entry to be valid, it must include a valid key, the current system clock value must be within the begin and end time of the key entry, and the algorithm specified in the key entry must be supported by the IS-IS protocol.

The IS-IS protocol supports the following algorithms:

• clear text password (RFC 5304 and RFC 5310 formats)

• HMAC-MD5 (RFC 5304 and RFC 5310 formats)

• HMAC-SHA-1 (RFC 5310 format)

• HMAC-SHA-256 (RFC 5310 format)

The IS-IS key entry may also include the option parameter to determine how the IS-IS protocol encodes the authentication signature. The value of basic results in the use of RFC 5304 format. The default or a value of isis-enhanced results in using the RFC 5310 format.

The error handling is described below.

• If a keychain exists but there are no active key entries with an authentication type that is valid for the associated protocol then inbound protocol packets are not authenticated and discarded and no outbound protocol packets should be sent.

• If keychain exists, but the last key entry has expired, a log entry is raised indicating that all keychain entries have expired. The IS-IS protocol requires that the protocol not revert to an unauthenticated state and requires that the old key is not to be used, therefore, after the last key has expired, all traffic is discarded.

IS-IS allows a two-level hierarchy to route PDUs. Level 1 areas can be interconnected by a contiguous level 2 backbone. The level 1 link-state database contains information only about that area. The level 2 link-state database contains information about the level 2 system and each of the level 1 systems in the area. A level 1/2 router contains information about both level 1 and level 2 databases. A level 1/2 router advertises information about its level 1 area toward the other level 1/2 or level 2 (only) routers.

Packets with destinations outside the level 1 area are forwarded toward the closest level 1/2 router which, in turn, forwards the packets to the destination area.

Sometimes, the shortest path to an outside destination is not through the closest level 1/2 router, or, the only level 1/2 system to forward packets out of an area is not operational. Route leaking provides a mechanism to leak level 2 information to level 1 systems to provide routing information about inter-area routes. Then, a level 1 router has more options to forward packets.

Configure a route policy to leak routers from level 2 into level 1 areas in the config>router>policy-options>policy-statement context.

The following example shows the command usage to configure prefix list and policy statement parameters in the config>router context.

config>router>policy-options# prefix-list  loops
..>policy-options>prefix-list# prefix 10.1.1.0/24 longer
..>policy-options>prefix-list# exit
..>policy-options# policy-statement leak
..>policy-options>policy-statement# entry 10
..>policy-options>policy-statement>entry# from
..>policy-options>policy-statement>entry>from# prefix-list loops
..>policy-options>policy-statement>entry>from# level 2
..>policy-options>policy-statement>entry>from# exit
..>policy-options>policy-statement>entry# to
..>policy-options>policy-statement>entry>to# level 1
..>policy-options>policy-statement>entry>to# exit
..>policy-options>policy-statement>entry# action accept
..>policy-options>policy-statement>entry>action# exit
..>policy-options>policy-statement>entry# exit
..>policy-options>policy-statement#exit
..>policy-options# commit
..>policy-options#

A:ALA-A>config>router>policy-options# info
----------------------------------------------
            prefix-list "loops"
                    prefix 10.1.1.0/24 longer
            exit
            policy-statement "leak"
                entry 10
                    from
                        prefix-list "loop"
                        level 2
                    exit
                    to
                        level 1
                    exit
                    action accept
                    exit
                exit
            exit
----------------------------------------------
A:ALA-A>config>router>policy-options# 

Next, apply the policy to leak routes from level 2 info level 1 systems on ALA-A.

config>router#isis
config>router>isis# export leak

A:ALA-A>config>router>isis# info
----------------------------------------------
        area-id 49.0180.0001
        area-id 49.0180.0002
        area-id 49.0180.0003
        authentication-key "//oZrvtvFPn06S42lRIJsE" hash
        authentication-type password
        no authentication-check
        export "leak"
...
----------------------------------------------
A:ALA-A>config>router>isis#

After the policy is applied, create a policy to redistribute external IS-IS routes from level 1 systems into the level 2 backbone (see Redistributing external IS-IS routers). In the config>router context, configure the following policy statement parameters:

config>router>policy-options# begin
..>policy-options# policy-statement "isis-ext"
..>policy-options>policy-statement# entry 10
..>policy-options>policy-statement>entry$ from
..>policy-options>policy-statement>entry>from$ external
..>policy-options>policy-statement>entry>from# exit
..>policy-options>policy-statement>entry# to
..>policy-options>policy-statement>entry>to$ level 2
..>policy-options>policy-statement>entry>to# exit
..>policy-options>policy-statement>entry# action accept
..>policy-options>policy-statement>entry>action# exit
..>policy-options>policy-statement>entry#  exit
..>policy-options>policy-statement# exit
..>policy-options# commit

A:ALA-A>config>router>policy-options# info
----------------------------------------------
            prefix-list "loops"
                    prefix 10.1.1.0/24 longer
            exit
            policy-statement "leak"
                entry 10
                    from
                        prefix-list "loop"
                        level 2
                    exit
                    to
                        level 1
                    exit
                    action accept
                    exit
                exit
            exit
            policy-statement "isis-ext"
                entry 10
                    from
                        external
                    exit
                    to
                        level 2
                    exit
                    action accept
                    exit
                exit
            exit
----------------------------------------------
A:ALA-A>config>router>policy-options#

IS-IS does not redistribute level 1 external routes into level 2 by default. You must explicitly apply the policy to redistribute external IS-IS routes. Policies are created in the config>router>policy-options context. See Route policies for more information.

The following example displays the policy statement configuration.

config>router>policy-options# info
----------------------------------------------
            prefix-list "loops"
                    prefix 10.1.1.0/24 longer
            exit
            policy-statement "leak"
                entry 10
                    from
                        prefix-list "loop"
                        level 2
                    exit
                    to
                        level 1
                    exit
                    action accept
                    exit
                exit
            exit
            policy-statement "isis-ext"
                entry 10
                    from
                        external
                    exit
                    to
                        level 2
                    exit
                    action accept
                    exit
                exit
            exit
----------------------------------------------
config>router>policy-options#

Specify the MAC address to use for all L1 or L2 IS-IS routers. The following example shows how to specify all L1 routers.

Example

- all-l1isis 01-80-C2-00-00-14

You can also specify the MAC address for all L2 IS-IS routers by using the all-l2isis command.