the same external route is not propagated back to the BGP/MPLS VPN backbone through
another PE-CE link.
Using Remote Neighbors to Configure OSPF Sham Links
When you employ OSPF as the PE-CE routing protocol in a BGP/MPLS VPN and also
configure OSPF backdoor links between VPN sites outside the backbone, the backdoor
links are always preferred over the backbone paths between the VPN links. OSPF sham
links prevent this problem, and you can implement them with OSPF remote neighbors.
Consider the topology shown in Figure 112 on page 482.
Figure 112: OSPF Topology with Backdoor Link
The PE routers are each running a separate logical OSPF instance for each VRF. Each of
these OSPF instances has adjacencies with their directly connected CE routers and
exchanges LSAs with those CE routers. The OSPF routes that are learned from a directly
connected CE router are installed into the IP routing table of the VRF associated with
that CE router.
The OSPF routes in the VRF’s IP routing table are then redistributed into MP-BGP and
advertised as VPNv4 routes to other PE routers. MP-BGP attaches extended communities
to the advertised routes to carry OSPF-specific attributes such as the route type and the
domain ID across the backbone.
At the remote PE router, the BGP routes are installed in the IP routing table of the VRF
and then redistributed back into the logical OSPF instance for that VRF. The remote PE
router uses the BGP extended communities to determine the type of LSA to send to CE
routers.
As a result the intra-area OSPF routes in one VPN site appear as interarea OSPF routes
at the remote VPN sites.
OSPF Backdoor Links
OSPF backdoor links typically serve as backup paths, providing a way for traffic to flow
from one VPN site to the other only if the path over the backbone is broken.
Copyright © 2010, Juniper Networks, Inc.482
JunosE 11.2.x BGP and MPLS Configuration Guide
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