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    Fast Reconvergence with Unique RDs

    You can assign a unique RD for the VRFs in each PE router to avoid the slow reconvergence issue. The route reflectors in the network consider advertised routes with different RDs to be different prefixes and therefore reflect both routes.

    In Figure 1, route reflector PE 4 reflects to PE 3 routes to the CE router through both PE 1 and PE 2. Suppose that the route through PE 1 is better than the route through PE 2. If you have assigned different RDs to the VRFs, then PE 4 reflects both routes to its client, PE 3.

    Figure 1: Topology for Fast Reconvergence by Means of Unique VRF RDs, Before Tunnels Go Down

    Topology for Fast Reconvergence by Means
of Unique VRF RDs, Before Tunnels Go Down

    If PE 1 goes down, the MPLS tunnels to it (PE 4–PE 1 and PE 3–PE 1) are dropped immediately. However, because the route reflector does not take into account the reachability of the next hop, it still reflects both the PE 1 route and the PE 2 route.

    When PE 3 imports these routes into its VRF, it resolves the routes and discovers that the tunnel to PE 1 is down. PE 3 declares the next hop for the route through PE 1 to be unreachable. It then selects the PE 2 route as the best route and installs it in the VRF’s IP routing table.

    On the other hand, if the VRFs in PE 1 and PE 2 share the same RD, the route reflector reflects only the best route, in this example the route through PE 1. If PE 1 goes down in this situation, PE 4 still reflects the route through PE 1. When PE 3 resolves the route, it finds that the tunnel is down and declares the next hop to be unreachable. Traffic then suffers a delay due to slow reconvergence.

    Assigning a unique RD for each VRF can be useful for other reasons as well:

    • PE-to-PE forwarding requires an MPLS tunnel from the ingress PE router to the egress PE router. In some topologies, such as networks with a sparse RSVP-TE mesh where the route reflector is not in the forwarding path, little correlation exists between the presence of an MPLS tunnel or IP connectivity from the route reflector to the egress PE router and the presence of the MPLS tunnel from the ingress PE router to the egress PE router.

      For these networks, relying on the ingress PE router is better than relying on the route reflector to decide which route is best. For this to work properly, the ingress PE router must be able to choose from all available paths, which in turn requires that each VRF have a unique RD.

    • If each VRF has a unique RD and the ingress PE router has all feasible paths to choose from, you can configure IBGP multipath and ECMP traffic over multiple PE-to-PE MPLS tunnels. This configuration is not possible if you use the same RD on multiple VRFs, because the ingress PE router in that case picks a single route that resolves to a single MPLS tunnel that is used end-to-end.

    Published: 2014-08-18