Juniper JUNOSE SOFTWARE FOR E SERIES 11.0.X - BGP AND MPLS CONFIGURATION GUIDE 2009-12-30 Configuration Manual page 485

Achieving Fast Reconvergence in VPN Networks
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:
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.
displayed with the show ip interface command. You must use the show ip
route or show ip static commands to display the interface.
If the interface in the parent VR goes down or is deleted, the static route added
in the VRF will continue to exist.
Example
host1(config-vrf)#ip route parent-router vr1stat
Use the no version to remove the static route.
See ip route.
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
Chapter 5: Configuring BGP-MPLS Applications
Configuring BGP VPN Services
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