Segment routing with BGP-LU prefix SID

Segment routing with BGP-LU prefix SID configurations

The example configurations that follow apply for AN-1 and ABR-2 to advertise a prefix SID for 10.0.0.1/32. Similar configurations (not shown) are required for AN-5 and ABR-4 to advertise a prefix SID for 10.0.0.5/32.

Enable segment routing for BGP on all nodes

The following example enables segment routing using the bgp segment-routing-mpls command.

--{ candidate shared default }--[  ]--
All-nodes# info network-instance default protocols bgp
    network-instance default {
        protocols {
            bgp {
                segment-routing-mpls {
                    admin-state enable
                }
            }
        }
    }

Associate local prefix SID label with the loopback interface on the access node (AN-1 shown)

The following example associates local prefix SID label 101 with the loopback interface lo0.0 on AN-1.

--{ candidate shared default }--[  ]--
AN-1# info interface lo0
    interface lo0 {
        admin-state enable
        subinterface 0 {
            ipv4 {
                address 10.0.0.1/32 {
                }
            }
        }
    }
--{ candidate shared default }--[  ]--
AN-1# info network-instance default interface lo0.0
    network-instance default {
        interface lo0.0 {
            interface-ref {
                interface lo0
                subinterface 0
            }
        }
    }
--{  candidate shared default }--[  ]--
AN-1# info network-instance default segment-routing
    network-instance default {
        segment-routing {
            mpls {
                local-prefix-sid 1 {
                    interface lo0.0
                    ipv4-label-index 101
                }
            }
        }
    }

Configure the ABR route table import policy (ABR-2 shown)

The following example defines a route table import policy for importing prefix 10.0.0.1/32 into the BGP RIB on ABR-2 using the routing-policy command. It also sets the bgp label-allocation prefix-sid reuse-igp action, which directs BGP-LU to use the IGP-signaled label-index to derive its advertised label. This has the effect of stitching the BGP segment routing tunnel to the IGP segment routing tunnel.

The example also applies the route table import policy to the BGP RIB on ABR-2 using the bgp rib-management command to populate the local BGP-LU table with the segment routing label for prefix 10.0.0.1/32.

--{ candidate shared default }--[  ]--
ABR-2# info routing-policy
    routing-policy {
        policy import-prefix-sid-2 {
            statement 10 {
                match {
                    prefix-set 10.0.0.1/32
                }
                action {
                    policy-result accept
                    bgp {
                        label-allocation {
                            prefix-sid {
                                reuse-igp true
                            }
                        }
                    }
                }
            }
        }
    }
--{ candidate shared default }--[  ]--
ABR-2# info network-instance default protocols bgp rib-management
    network-instance default {
        protocols {
            bgp {
                rib-management {
                    table ipv4-labeled-unicast {
                        route-table-import import-prefix-sid-2
                    }
                }
            }
        }
    }

Configure the export routing policy for the ABRs (ABR-2 shown)

The following example defines a policy for ABR-2 to export prefix 10.0.0.1/32 to BGP neighbors using the routing-policy command. It applies the policy on ABR-2 using the bgp neighbor command. It also configures BGP group iBGP with BGP-LU enabled, and applies the group to the peer.

The policy advertises the segment routing label for prefix 10.0.0.1/32 from the ABR-2 BGP-LU table to neighbor P-3 (192.0.2.3).

--{ candidate shared default }--[  ]--
ABR-2# info routing-policy
    routing-policy {
        policy export-prefix-sid-1 {
            statement 10 {
                match {
                    prefix-set 10.0.0.1/32
                }
                action {
                    policy-result accept
                }
            }
        }
    }
--{ candidate shared default }--[  ]--
ABR-2# info network-instance default protocols bgp
    network-instance default {
        protocols {
            bgp {
                group iBGP {
                    afi-safi ipv4-labeled-unicast {
                        admin-state enable
                    }
                }
                neighbor 192.0.2.3 {
                    peer-as 64496
                    peer-group ABRs
                    export-policy [
                        export-prefix-sid-1
                    ]
                }
            }
        }
    }

Block propagation of prefix SIDs

By default, the prefix SID attribute propagates without restriction. To prevent the prefix SID from propagating outside the segment routing domain where it is applicable, use the network-instance protocols bgp [group | neighbor] optional-attributes block-prefix-sid command. The command removes the prefix SID attribute from all routes sent to and received from the iBGP and eBGP peers included in the scope of the command.

The following example blocks the propagation of prefix SIDs to and from BGP neighbor 192.0.2.10 (not shown in the topology diagram).

--{ + candidate shared default }--[  ]--
Any-node# info network-instance default protocols bgp neighbor 192.0.2.10
    network-instance default {
        protocols {
            bgp {
                neighbor 192.0.2.10 {
                    optional-attributes {
                        block-prefix-sid true
                    }
                }
            }
        }
    }

Diplay the prefix SID path attribute

To display the path attributes of the advertised BGP-LU routes, including the prefix-sid attribute, use the info from state bgp-rib attr-sets attr-set command. The following example shows the BGP-LU path attributes advertised from ABR-2 to P-3. In this example, the info from state command is entered from the network-instance context.

--{ candidate shared default }--[ network-instance default ]--
P-3# info from state bgp-rib attr-sets attr-set <attr-id>
    origin igp
    atomic-aggregate false
    next-hop 192.0.2.2
    med 0
    local-pref 100
    prefix-sid {
        tlv label-index {
            label-index {
                label-index 101
            }
        }
        tlv srgb-originator {
            srgb-originator {
                srgb [
                    16000:1000
                ]
            }
        }
    }