Fast Reconvergence by Means of Reachability Checking

You might not want to assign different RDs for each VRF in some circumstances, such as the following:

You can use the check-vpn-next-hops command to avoid the slow reconvergence problem without having to configure a unique RD for each VRF. When you issue this command, BGP verifies the reachability of the next hop on VPNv4 routes received from MP-IBGP peers before it imports those routes into a VRF. This behavior enables the VPNv4 route reflectors to take into account the reachability of the next hop when they select the best route to reflect.

Consider a topology similar to that discussed in the previous section. As before, the route through PE 1 is considered to be the best. VRFs share the same RD, but reachability checking has been enabled. In Figure 101, PE 1 has already failed, and tunnels PE 3–PE 1 and PE 4–PE 1 have gone down.

Figure 101: Topology for Fast Reconvergence by Means of Reachability Checking, After Tunnels Go Down

Topology for Fast Reconvergence by Means
of Reachability Checking, After Tunnels Go Down

When the MPLS tunnel (RSVP-TE or LDP) to the next hop of the best route goes down, the VPNv4 route reflector immediately advertises the next-best route (if any) without waiting for the MP-IBGP session to go down. In this example, that route is through PE 2.

This command is available only in the context of the VPNv4 unicast and VPNv6 unicast address families. The behavior is the same for both address families.

Use the show ip bgp vpnv4 all summary or show bgp ipv6 vpnv6 all summary command to view the status of next hop reachability checking.

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