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.
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.
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