SR P2MP policy

The Segment Routing (SR) Point-to-Multipoint (P2MP) policy removes the need for the traditional underlay signaling layers like multipoint LDP (mLDP) and P2MP RSVP-TE. A P2MP policy can be instantiated statically using the CLI on the Path Computation Element Client (PCC), or instantiated dynamically using a Path Computation Element (PCE). The PCE uses the Path Computation Element Protocol (PCEP) to program the PCC.

The P2MP policy datapath and forwarding plane use MPLS instructions, similar to mLDP and P2MP RSVP-TE, and are programmed using a replication segment object. The replication segment is a forwarding entity with an incoming label and a set of Outgoing Interfaces (OIF) and labels. A P2MP policy can be used in a Next-Generation Multicast VPN (NG-MVPN) as a provider tunnel.

This functionality is described in the following IETF drafts:

  • draft-voyer-pim-sr-p2mp-policy

  • draft-voyer-spring-sr-replication-policy

  • draft-dhs-spring-sr-p2mp-policy-yang

  • draft-hsd-pce-sr-p2mp-policy

  • draft-hb-idr-sr-p2mp-policy

SR P2MP policy details

A P2MP policy represents a multicast tree from the root node to a set of leaf nodes and is a single provider tunnel. A P2MP policy can contain redundant trees from the root to leaf nodes, each with its own preference. This redundancy is implemented using Candidate Paths (CPs). Each CP represents a P2MP tree with its own Traffic Engineering (TE) constraints. The CPs can be optimized based on link failures or IGP optimizations. Each CP can contain multiple P2MP LSPs represented by path instance IDs. The CP can perform make-before-break between these path instances (P2MP LSPs).

A P2MP policy is relevant only on the root node where the P2MP tree is instantiated. The P2MP policy, which is identified by the tuple <root ID, tree ID>, does not include forwarding information for the P2MP LSP. The policy only contains information about the root and leaf nodes and the TE, which is required to set up the tree from the root to the leaf nodes. The forwarding information is part of the replication segment. The root, transit, and leaf nodes contain replication segments.

Replication segment

A replication segment is the forwarding instruction for a P2MP LSP. It contains the incoming replication SID and a set of OIFs and their corresponding SID or SID list. A replication segment is identified by a tree ID, root ID, and path instance ID (LSP ID) through the root, transit, and leaf nodes.

A second kind of replication segment is a shared replication segment. A shared replication segment is shared between multiple root nodes or P2MP LSPs. For this reason, it does not have a root ID, but contains a replication segment identifier, which is within the tree ID. A shared replication segment can be used for Fast Reroute (FRR). Currently, the P2MP policy supports a link protection facility bypass FRR.

P2MP and replication segment objects

In the following figure, nodes A, C, D, and E are replication-segment capable and B is unicast SR capable (that is, B is not replication-segment capable).

Figure 1. P2MP and replication segment objects

Multiple CPs can exist under a P2MP policy. The CPs act as tree redundancy. There can be only one active CP in the P2MP policy, based on the CP preference. The highest CP preference is the active CP.

Multiple path instances can exist under a CP. Each path instance is a P2MP LSP and each instance is presented with an instance ID. Path instances can be used for global optimization of the active CP. Each path instance is built using replication segments, which forward P2MP tree information through the network at the root, transit, and leaf nodes. The P2MP policy is correlated to its replication segment by its root ID, tree ID, and instance ID.

The replication segments forward information with one or more OIFs to replicate and forward the PDUs. On the transit and leaf nodes, the incoming replication SID identifies the replication segment and its forwarding information. The replication segment can also contain FRR information for each of the outgoing interfaces.

The two replication segments on router A and C can be connected to each other using a unicast SR policy. To do so, the replication segment on router A is programmed with a SID list. The replication SID of router C is at the bottom of the stack and the SR labels connecting A to C are on top of the stack (that is, adjacency SIDs or node SIDs).

In Packet representation of a multicast stream, node B is not replication-policy capable, so node A pushes the SID list of B and C, where C is the replication SID at the bottom of the stack and B is the node SID.

Figure 2. Packet representation of a multicast stream

SR P2MP policy instantiation

The SR P2MP policy can be instantiated either statically on the PCC using the CLI, or dynamically using a PCE.

SR P2MP policy instantiation using the CLI

The CLI can be used to configure a P2MP policy, its CPs, and path instances on the root node. The CLI can also be used to create the replication segments on the root, transit, and leaf nodes. The P2MP policy can be assigned to NG-MVPN Inclusive P-Multicast Service Interfaces (IPMSIs) and Selective PMSIs (SPMSIs).

On each node, each replication segment represents a unique P2MP LSP with the following key: <tree ID, root ID, instance ID>. The instance ID and tree ID are unique to each root.

SPMSI for static P2MP policy

For a static P2MP policy, a single P2MP policy is assigned to an SPMSI. All the (S,G)s that are required to switch to the SPMSI and send an SPMSI AD route use this single P2MP policy. To assign (S,G)s to a different P2MP policy, use multi-stream SPMSIs and assign different (S,G)s to different SPMSIs.

PMSI tree ID advertised by BGP

The tree ID used in MP-BGP to advertise the AD routes is inherited from the P2MP policy assignment to the provider tunnels; it is not generated automatically.

SR P2MP policy instantiation using PCE

NG-MVPN can be configured using the CLI or SNMP on the PCCs.

The root node discovers all of the leaf nodes through NG-MVPN. The root and leaf nodes information is updated to the PCE using PCEP.

The PCE calculates the shortest path from the root to the leaf nodes and takes into account any programmed constraints. The PCE has an end-to-end view of the network through BGP LS.

After calculating the tree, the PCE downloads the P2MP policy to the root and the replication policies to the root, transit, and leaf nodes.

Updates to the P2MP policy or the replication paths are calculated by the PCE and downloaded accordingly.

SPMSI for PCE P2MP policy

When the root node listens to the MP-BGP SPMSI AD routes and determines that a set of (S,G)s are interested in an SPMSI, it sends an update message to the PCE with the <tree ID, root ID> of the SPMSI AD route and the leaf that is interested in joining this SPMSI. The PCE uses this information to build a P2MP policy for that specific tree ID and downloads it to be used for an SPMSI.

Configuring an MVPN service with SR P2MP policies for SPMSI and IPMSI

This section provides examples to configure an MVPN service with SR P2MP policies for SPMSIs and IPMSIs. The configuration examples use the following network.

node-2-(100.101.1.x/24)---P-(101.102.1.x/24)----Leaf-1
|--(101.103.1.x/24)----Leaf-2

The following are the network settings:

  • Node-2 is 100.0.0.100.
  • P is 100.0.0.101.
  • Leaf-1 is 100.0.0.102.
  • Leaf-2 is 100.0.0.103.

The following example show a reserved label block configured (in this example, ‟treeSID”), from which the tree SID labels are allocated.

Configure a reserved label block for MPLS labels (MD CLI)

[ex:/configure router "Base" mpls-labels]
A:admin@node-2# info
    reserved-label-block "treeSID" {
        start-label 30000
        end-label 30999
    }

Configure a reserved label block for MPLS labels (classic CLI)

A:node-2>config>router>mpls-labels# info 
----------------------------------------------
            reserved-label-block "treeSID"
                start-label 30000 end-label 30999
            exit

The following example shows the reserved label block enabled for SR P2MP.

Enable a reserved label block for SR P2MP (MD-CLI)

[ex:/configure router "Base" p2mp-sr-tree]
A:admin@node-2# info
    reserved-label-block "treeSID"

Enable a reserved label block for SR P2MP (classic CLI)

A:node-2>configure>router>p2mp-sr-tree# info 
----------------------------------------------
        reserved-lbl-block "treeSID"

The following example shows a P2MP policy configured with a tree on the node-2 PE. Two policies are configured, one for an IPMSI and one for an SPMSI.

Configure a P2MP policy on the node with associated CP and path instance (MD-CLI)

[ex:/configure router "Base" p2mp-sr-tree]
A:admin@node-2# info
    ...
    p2mp-policy "IPMSI-VPRN1" {
        admin-state enable
        node-2-address 100.0.0.100
        tree-id 9000
        candidate-path "Primary-path" {
            admin-state enable
            active-instance 1
            preference 1000
            path-instances 1 {
                instance-id 1000
            }
        }
    }
    p2mp-policy "SPMSI-VPRN1" {
        admin-state enable
        node-2-address 100.0.0.100
        tree-id 9000
        candidate-path "Primary-path" {
            admin-state enable
            active-instance 1
            preference 1000
            path-instances 1 {
                instance-id 1000
            }
        }
    }

Configure a P2MP policy on the node with associated CP and path instance (classic CLI)

A:node-2>config>router>p2mp-sr-tree# info 
----------------------------------------------
        ...
        p2mp-policy "IPMSI-VPRN1"
            node-2-address 100.0.0.100
            node-2-tree-id 9000
            p2mp-candidate-path "Primary-path"
                preference 1000
                instances
                    instance 1 instance-value 1000
                exit
                active-instance 1
                no shutdown
            exit
            no shutdown
        exit
        p2mp-policy "SPMSI-VPRN1"
            node-2-address 100.0.0.100
            node-2-tree-id 9001
            p2mp-candidate-path "Primary-path"
                preference 1000
                instances
                    instance 1 instance-value 1000
                exit                  
                active-instance 1
                no shutdown
            exit
            no shutdown
        exit

The following examples shows the P2MP policies assigned to NG-MVPN on the node. BGP must be established between the node-2 and leaf routers using IPv4/IPv6 MVPN Assured Forwarding (AF). The tree ID configured in the P2MP policies is used to advertise in the BGP Provider Tunnel Attribute (PTA) field.

Assign P2MP policies to NG-MVPN (MD-CLI)

[ex:/configure service vprn "1"]
A:admin@node-2# info
    customer "1"
    interface "to14" 
    pim {
        rp {
            ipv4 {
                bsr-candidate {
                admin-state enable
            }
                rp-candidate {
                admin-state enable
            }
        }
    }
    mvpn {
        c-mcast-signaling bgp
        auto-discovery {
            type bgp
        }
        vrf-target {
            unicast true
        }
        provider-tunnel {
            inclusive {
                p2mp-sr {
                    static-policy "IPMSI-VPRN1"
                    admin-state enable
                }
            }
            selective {    
                data-threshold {
                    group-prefix 231.0.0.0/24 {
                        threshold 10
                    }
                }
                p2mp-sr {
                    static-policy "SPMSI-VPRN1"
                    admin-state enable
                }
                data-threshold 231.0.0.0/24 10
               
            }
        }
    }
    bgp-ipvpn {
        mpls {
            admin-state enable
            route-distinguisher "70:70"
            vrf-target {
                community "target:70:70"
            }
            auto-bind-tunnel {
                resolution filter
                resolution-filter {
                    sr-isis true
                }
            }
        }
    }

Assign P2MP policies to NG-MVPN (classic CLI)

A:node-2>config>service>vprn# info 
----------------------------------------------
            route-distinguisher 70:70
            auto-bind-tunnel
                resolution-filter
                    sr-isis
                exit
                resolution filter
            exit
            vrf-target target:70:70
            interface "to14" create
            exit
… 
            pim
                rp
                    static
                    exit
                    bsr-candidate     
                        no shutdown
                    exit
                    rp-candidate
                        no shutdown
                    exit
                exit
                no shutdown
            exit
            mvpn
                auto-discovery default
                c-mcast-signaling bgp
                provider-tunnel
                    inclusive
                        p2mp-sr
                            static-policy " IPMSI-VPRN1"
                            no shutdown
                        exit
                    exit
                    selective
                        p2mp-sr
                            static-policy " SPMSI-VPRN1"
                            no shutdown
                        exit          
                        data-threshold 231.0.0.0/24 10
                    exit
                exit
                vrf-target unicast
                exit
            exit
            no shutdown

You must configure the corresponding replication segment and forwarding instructions on the node, transit, and leaf nodes for the P2MP policies. The following example shows the configuration on the node.

Configure replication segment and forwarding instructions on the node (MD-CLI)

[ex:/configure router "Base" p2mp-sr-tree]
A:admin@node-2# info   
    ...
    replication-segment "IPMSI-VPRN1-NODE-2" {
        admin-state enable
        instance-id 1000
        node-2-address 100.0.0.100
        tree-id 9000
        sid-action push
        downstream-nodes 1 {
            admin-state enable
            next-hop-address "100.101.1.2"
            label {
                sid-list 1 {
                    replication-sid 30000
                }
            }
        }
    }
    replication-segment "SPMSI-VPRN1-node-2" {
        admin-state enable
        instance-id 1000
        node-2-address 100.0.0.100
        tree-id 9001
        sid-action push
        downstream-nodes 1 {
            admin-state enable
            next-hop-address "100.101.1.2"
            label {
                sid-list 1 {
                    replication-sid 30001
                }
            }
        }
    }

Configure the replication segment and forwarding instructions on the node (classic CLI)

A:node-2>configure>router>p2mp-sr-tree# info 
----------------------------------------------
        ...
        replication-segment "IPMSI-VPRN1-NODE-2"
            node-2-address 100.0.0.100
            node-2-tree-id 9000
            sid-action push
            instance-id 1000
            next-hop-id "1"
                next-hop-address 100.101.1.2
                replication-sid 30000
                no shutdown
            exit
            no shutdown
        exit
        replication-segment "SPMSI-VPRN1-NODE-2"
            node-2-address 100.0.0.100
            node-2-tree-id 9001
            sid-action push
            instance-id 1000
            next-hop-id "1"
                next-hop-address 100.101.1.2
                replication-sid 30001
                no shutdown
            exit
            no shutdown
        exit

The following example shows the configuration of the replication segment for IPMSI on the P router follows. The SPMSI replication router on the P router is configured in the same way, but with a tree ID of 9001 and an incoming SID of 30001.

Configure a replication segment for IPMSI on the P router (MD-CLI)

[ex:/configure router "Base" p2mp-sr-tree]
A:admin@node-2# info
    admin-state enable
    reserved-label-block "treeSID"
    replication-segment "rs-IPMSI-VPRN1-100.0.0.100" {
        admin-state enable
        instance-id 1000
        replication-sid 30000
        node-2-address 100.0.0.100
        tree-id 9000
        sid-action swap
        downstream-nodes 1 {
            admin-state enable
            # toleaf1
            next-hop-address "101.102.1.2"
            label {
                sid-list 1 {
                    replication-sid 30000
                }
            }
        }
        downstream-nodes 2 {
            admin-state enable
            # toleaf2
            next-hop-address "101.103.1.2"
            label {
                sid-list 1 {
                    replication-sid 30000
                }
            }
        }
    }

Configure a replication segment for IPMSI on the P router (classic CLI)

A:node-2>configure>router>p2mp-sr-tree# info 
----------------------------------------------
        reserved-lbl-block "treeSID"
        replication-segment "rs-IPMSI-VPRN1-100.0.0.100"
            node-2-address 100.0.0.100
            node-2-tree-id 9000
            sid-action swap
            incoming-sid 30000
            instance-id 1000
            next-hop-id "1"
                next-hop-address 101.102.1.2 //toleaf-1
                replication-sid 30000
                no shutdown
            exit
            next-hop-id "2"
                next-hop-address 101.103.1.2 //toleaf-2
                replication-sid 30000
                no shutdown
            exit
            no shutdown
        exit

The following example shows the replication segment on the leaf-1 router.

Configure a replication segment for the leaf-1 router (MD-CLI)

[ex:/configure router "Base" p2mp-sr-tree]
A:admin@node-2# info
    admin-state enable
    reserved-label-block "treeSID"
    replication-segment "rs-IPMSI-VPRN1-100.0.0.100" {
        admin-state enable
        instance-id 1000
        replication-sid 30000
        node-2-address 100.0.0.100
        tree-id 9000
        sid-action pop
    }

Configure a replication segment for the leaf-1 router (classic CLI)

A:node-2>configure>router>p2mp-sr-tree# info 
----------------------------------------------
        reserved-lbl-block "treeSID"
        replication-segment "rs-IPMSI-VPRN1-100.0.0.100"
            node-2-address 100.0.0.100
            node-2-tree-id 9000
            sid-action pop
            incoming-sid 30000
            instance-id 1000
            no shutdown
        exit

The following example shows the replication segment on the Leaf-2 router.

Configure a replication segment for the leaf-2 router (MD-CLI)

[ex:/configure router "Base" p2mp-sr-tree]
A:admin@node-2# info
    admin-state enable
    reserved-label-block "treeSID"
    replication-segment "rs-IPMSI-VPRN1-100.0.0.100" {
        admin-state enable
        instance-id 1000
        replication-sid 30000
        node-2-address 100.0.0.100
        tree-id 9000
        sid-action pop
    }

Configure a replication segment for the leaf-2 router (classic CLI)

A:node-2>configure>router>p2mp-sr-tree# info 
----------------------------------------------
        reserved-lbl-block "treeSID"
        replication-segment "rs-IPMSI-VPRN1-100.0.0.100"
            node-2-address 100.0.0.100
            node-2-tree-id 9000
            sid-action pop
            incoming-sid 30000
            instance-id 1000
            no shutdown
        exit

Administrative behavior of tree SID

CP selection criteria

The active CP is chosen as follows:

  • The higher value protocol origin is selected.

  • The existing installed path is preferred.

  • The lower value of originator is selected.

  • The higher value of discriminator is selected.

The CLI static configuration has a protocol origin value of 30. This value is not configurable.

The CLI originator value is <0, 0.0.0.0>.

Based on the CLI configuration, the CP selection is as follows:

  • If the P2MP policy is operational and has an operational CP, the following handling applies.

    • If a CP with a higher preference is configured and becomes operational, the users should switch to it.

    • If the current CP is the highest preference and another CP with the same preference is configured, the users should stay on the current CP.

  • If you administratively disable and then enable the P2MP policy, and there are multiple CPs with the same preference, select the CP that was created most recently. The show command for the CP displays the creation time.

CP operational status

The CP operational status changes to down if at least one of the following conditions becomes true.

  • The CP has no active instance.

  • The CP is shut down.

  • The replication segment correlating to the CP on the root is operationally down or does not exist.

P2MP policy operational status

If at least one of the following conditions becomes true, the operational status of the P2MP policy and the status of the PMSI corresponding to the P2MP policy change to down.

  • All the CPs in the P2MP policy are operationally down.

  • The P2MP policy is administratively disabled.

Replication segment operational status

A replication segment is operationally down when one of the following is true:

  • all its next-hops (NHs) are operationally down, including the FRR NHs.

  • no NHs are configured

The replication segment is operationally up if at least one NH is operationally up on the replication segment.

FRR behavior

Only facility bypass and link protection are supported for a P2MP policy. Node protection is not supported. The facility bypass can be created using shared replication segments. Shared replication segments do not have a tree ID. They are identified using a replication segment identifier within the tree ID.

At the Point of Local Repair (PLR), the primary replication segment has a protection next hop. This protection next-hop has a second OIF with its own outgoing label, which is used for the facility protection tunnel.

The facility protection tunnel can consist of multiple transit nodes until the tunnel reaches the Merge Point (MP). Replication segments are configured on these transit nodes to complete the facility protection tunnel. Multiple P2MP trees can share the facility protection tunnel at the PLR. The facility protection tunnel uses the implicit null label (see Implicit null case) or an actual label at the Penultimate Hop Popping (PHP) node (see Non-implicit null case).

Implicit null case

If the implicit null label is used, the protection tunnel label is popped and the tree SID P2MP LSPs are forwarded to the MP with the tree SID label.

In Protection using the implicit null label, node A is protecting link A-B through node C, so the facility protection tunnel is set up through node C. Node C is a PHP node and is programmed to swap the facility protection label with implicit null (label 3).

After a failure on the A-B link, node A pushes label 2000 as indicated in the protection next hop programmed in the replication segment for trees 1 and 2. Trees 1 and 2 share the same facility protection tunnel (label 2000). The packet is forwarded to node C, where node C pops the protection next hop and forwards the packet to node B with a replication SID on top of the packet.

Figure 3. Protection using the implicit null label

Non-implicit null case

If an implicit null label on the PHP node is not used, a replication segment is needed on the MP to pop the facility protection label and forward the underlying traffic based on the tree SID label.

In Protection using the actual label at the PHP node, node A is protecting the A-B link through node C, so the facility protection tunnel is set up through node C. Node C is a PHP node and is programmed to swap the facility protection label with label 3000.

After a failure on link A-B, node A pushes label 2000 as indicated by the protection next-hop programmed in the replication segment for trees 1 and 2. The packet is forwarded to node C, where node C swaps the protection label with label 3000 for both tree 1 and tree 2, and forwards the packet to node B. Node B has a replication segment for the facility protection tunnel, which has an action of pop 3000. After popping 3000 on node B, the tree SID label for label 1 and label 2 is exposed and the corresponding replication segment is found and executed.

Figure 4. Protection using the actual label at the PHP node

FRR recovery behavior

After the primary path is recovered, the P2MP LSP switches back to the primary path and away from the protection next-hop (FRR). This switch back to primary may cause a brief traffic outage.

The replication SID next-hops send optimistic ARP to populate the ARP table. If the next-hop MAC address is not found in the ARP table (that is, the address is not populated by BFD, IGP, or any other packet), the optimistic ARP populates the ARP table. The reversion from FRR to the primary next-hop happens only if the primary path ARP entry is found in the ARP table.

BFD behavior

BFD can be enabled under the p2mp-sr-tree context for the replication segment next-hops. BFD is enabled at the P2MP SR tree level. The P2MP SR tree registers itself with the current available BFD session. The BFD sessions need to be enabled using other protocols. For example, static route, OSPF, or IS-IS can enable the BFD session on an interface. The replication segments are registered with these BFD sessions. The replication segments cannot initiate a BFD session and rely on other protocols to initiate the BFD session because a replication segment is a unidirectional entity, while BFD is a bidirectional protocol.

When BFD is enabled under the p2mp-sr-tree context, all replication next hops that are using a Layer 3 interface with BFD enabled on that interface register with the BFD module. If the BFD status on the Layer 3 interface goes down, any replication segment next-hop that is using that Layer 3 interface goes operationally down. This operationally down status of the next-hop within a replication segment can cause an FRR.

Only single hop BFD is supported. BFD for unnumbered interfaces is not supported.

For IPv6, protocols such as OSPF or LDP create a BFD session to the link local interface. A static route can create a BFD session to link local or global IPv6 addresses. To use BFD for IPv6 next hops within a replication segment, the replication segment needs to be configured with a link local next-hop for protocols that create the BFD session to the link local address. This way, the replication segment next hop finds a BFD session created by one of these protocols.

Maximum SPMSI behavior

The maximum P2MP SPMSI value configured under an MVPN selective provider tunnel does not affect any established SPMSIs. It only affects new spawning SPMSI counts.

If any existing SPMSIs are above the maximum P2MP SPMSI threshold, no new SPMSIs are spawned until the number of SPMSIs goes below the threshold.

Global optimization of P2MP policy and MBB behavior

Global optimization of the P2MP policy candidate path is supported, in addition to local FRR, where the protection next hop is downloaded by the replication segment and the FRR action is triggered by a port failure or BFD failure.

To use the global optimization behavior, the user creates another instance under the P2MP policy. The appropriate replication segments must be created for this optimized instance also. After the entire tree is created, the active instance under the CP can be set to this new, optimized instance and a switch from the previous instance to this optimized instance is performed. While the switch is in progress, the MVPN on the leaf is accepting traffic from both instances of the CP. After the switch is complete, the old instance can be deleted from the candidate path and its replication segments can be removed.

Global optimization of PCEP behavior

Global optimization is supported on the PCE. Local optimization of a replication segment using PCEP is not supported. If the PCE calculates an optimized path for a candidate path, that path instance is different from the current path instance. For this reason, a candidate path contains two path instances. The PCE must download a new path instance with an LSP ID of 0 and the PLSP ID of the current candidate path. This behavior applies to replication segments only.

When the current path instance is modified from the PCE to the PCC, the PCC-assigned LSP ID and PLSP ID are sent from the PCE to the PCC. This behavior ensures that the LSP ID of the replication segment for the existing path instances does not change.

PCEP behavior

You can clear all states on the PCC, including replication segments and P2MP policies. The state of the P2MP LSP on the PCC is operationally up as long as there is one valid OIF and is operationally up for that LSP.

Use the following command to clear all states on the PCC:

  • MD-CLI
    configure router p2mp-sr-tree admin-state disable
  • classic CLI
    configure router p2mp-sr-tree shutdown

PCE pop with next-hop 127.0.0.0/8 or ::1

For a PCE, to program the datapath with a pop action, the next hops must be programmed as 127.0.0.0/8 or IPv6 ::1. If the replication segment next-hop has no information, the next hop is reported to the PCE with status down.

For CLI-initiated replication segments, the next-hop label action can be set to pop, and the next hop does not need to be programmed as 127.0.0.0/8 or ::1.

P2MP policy special considerations

For interfaces that are not configured as unnumbered, the next-hop used in the replication segment must be a direct (local) next-hop. The replication segment cannot resolve indirect next-hops to a downstream router loopback or system IP address.

The FRR outage time can exceed 50 ms when a node has a large number of replication segments that are using a protection tunnel for FRR.

Replication segment steering through a unicast SR network

When a unicast SR network is present between two replication SIDs, it is possible to connect the two replication SIDs through a unicast SID list, as shown in Replication segment steering through a unicast SR network. The SID list can be a list of adjacency or node SIDs that provides a traffic-engineered path though the unicast domain to connect the two replication SIDs.

Figure 5. Replication segment steering through a unicast SR network

The unicast SID list can be configured by listing the node or adjacency SIDs under a replication segment. Even if two replication segments are connected directly, the egress interface or the next hop can be programmed using a SID list. For example, an adjacency SID can be used as a mechanism to steer the packet out of a local interface.

The unicast fast reroute functionality and Equal Cost Multipath (ECMP) functionality are not available on the egress replication segment node on which the SID list is configured. However, the next downstream node in the unicast SR domain can take advantage of all unicast SR TE and resiliency features, for example, Loop-Free Alternate (LFA), Remote Loop-Free Alternate (RLFA), or Topology-Independent Loop-Free Alternate (TI-LFA).

On the egress replication segment node, the protection next hop can be configured using a replication SID. In addition, a SID list can be used in any next-hop object under the replication segment, including the protection next-hop object, as shown in Primary and protection paths with SID list.

Figure 6. Primary and protection paths with SID list