BGP shortcuts configuration over segment routing tunnels

When you have segment routing enabled in your network, you can specify a segment routing tunnel as the next hop for a BGP route. This capability is referred to as BGP shortcuts over segment routing.

SR-ISIS-IPv4 tunnels in TTM can resolve BGP IPv4 routes.

SR-ISIS-IPv6 tunnels in TTM can resolve BGP IPv4 and BGP IPv6 routes.

To configure BGP shortcuts, you must configure the BGP protocol with the allowed tunnel types and the required tunnel resolution mode.

Configuring BGP shortcuts over segment routing

  1. In the default network instance, define the tunnel-resolution mode for the BGP protocol.
    This setting determines the order of preference and the fallback when using tunnels in the tunnel table instead of routes in the FIB. Available options are as follows:
    • require

      requires tunnel table lookup only

    • prefer

      prefers tunnel table lookup over FIB lookup

    • disabled (default)

      performs FIB lookup only

  2. Set the allowed tunnel types for next-hop resolution.

Configure IPv4 BGP shortcuts

The following example shows the BGP next-hop resolution configuration to allow IPv4 SR-ISIS tunnels, with the tunnel mode set to prefer. 

--{ * candidate shared default }--[ ]--
# info network-instance default protocols bgp afi-safi ipv4-unicast ipv4-unicast next-hop-resolution ipv4-next-hops tunnel-resolution
    network-instance default {
        protocols {
            bgp {
                afi-safi ipv4-unicast {
                    ipv4-unicast {
                        next-hop-resolution {
                            ipv4-next-hops {
                                tunnel-resolution {
                                    mode prefer
                                    allowed-tunnel-types [
                                        sr-isis
                                    ]
                                }
                            }
                        }
                    }
                }
            }
        }
    }

Configure IPv6 BGP shortcuts

The following example shows the BGP next-hop resolution configuration to allow IPv6 SR-ISIS tunnels, with the tunnel mode set to prefer. 

--{ * candidate shared default }--[ ]--
# info network-instance default protocols bgp afi-safi ipv6-unicast ipv6-unicast next-hop-resolution ipv6-next-hops tunnel-resolution
    network-instance default {
        protocols {
            bgp {
                afi-safi ipv6-unicast {
                    ipv6-unicast {
                        next-hop-resolution {
                            ipv6-next-hops {
                                tunnel-resolution {
                                    mode prefer
                                    allowed-tunnel-types [
                                        sr-isis
                                    ]
                                }
                            }
                        }
                    }
                }
            }
        }
    }