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    Fast Reconvergence in VPN Networks Overview

    By default, BGP does not confirm the reachability of the BGP indirect next hop of VPNv4 routes received over an MP-IBGP session until those routes have been imported into a VRF.

    To BGP, the next hops of VPNv4 routes that are still in the global VPNv4 table (viewable with the show ip bgp vpnv4 all command) are always reachable. As a result, VPNv4 route reflectors that have multiple paths to the same prefix select the best route to reflect without taking into account the reachability of the BGP indirect next hop. Instead, best-path selection is based on weight, local preference, AS-path length, and other attributes.

    After the route has been imported into a VRF, the reachability of the BGP indirect next hop is based on the presence of an MPLS tunnel (LDP or RSVP-TE) to the next-hop address and not on the presence of an IP route to the next-hop address.

    Disregarding the reachability of the BGP indirect next hop when the router selects the best route to reflect can cause very slow reconvergence (up to 90 seconds) after a topology change in BGP/MPLS VPN networks that match all of the following conditions:

    • Have a full mesh of LDP MPLS tunnels
    • Have multihomed CE routers
    • Use the same RD for multiple VRFs
    • Rely on VPNv4 route reflectors as arbiters for selecting the best VPNv4 route from a set of clients

    If a PE router fails in such a network, the route reflector must quickly reflect the VPNv4 route from the next-best PE router without having to wait for the BGP session to the failed PE router to time out. Depending on the network topology, you can achieve fast reconvergence by assigning unique RDs to each VRF or by enabling next-hop reachability checking.

    Published: 2014-08-18