Juniper JUNOSE 11.2.X BGP AND MPLS, Configuration Manual

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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.
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:
•
Allocating a unique RD for each VRF might be an administrative burden.
•
If the network is already in operation and configured with the same RD for all VRFs in
a given VPN, then changing the RDs might affect service.
• Each route reflector might act as an arbiter for a geographic area and be responsible
for maintaining a list of all feasible paths to egress PE routers that can be used to reach
a given prefix. Because the route reflector selects only one best path and reflects that
single best path toward its clients and nonclients, the amount of state in the network
is reduced. The core of the network and other geographic areas need only the one best
route to each prefix in a given remote geographical area.
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 100 on page 457, PE 1 has already failed, and tunnels
PE 3–PE 1 and PE 4–PE 1 have gone down.
Figure 100: Topology for Fast Reconvergence by Means of Reachability Checking, After Tunnels
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457 Copyright © 2010, Juniper Networks, Inc.
Chapter 6: Configuring BGP-MPLS Applications