Juniper JUNOSE 11.2.X BGP AND MPLS, Configuration Manual

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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 99 on page 456, 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 99: 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
Copyright © 2010, Juniper Networks, Inc. 456
JunosE 11.2.x BGP and MPLS Configuration Guide