OSPF Sham Link

If a VRF contains both an OSPF-distributed route and a VPN-IPv4 route for the same IPv4 prefix, then the backdoor OSPF-distributed route is preferred over the VPN backbone route, unless the next hop interface for an installed (OSPF distributed) route is the sham link, in which case, the VPN backbone VPN-IPv4 route is used.

If it is desired to have OSPF prefer the routes through the VPN backbone over the routes through the OSPF backdoor link, then the routes through the backbone must appear to be intra-area routes. The sham link provides this appearance of an intra-area link connecting the two PE routers.

Use the area sham-link command in OSPF configuration mode, specifying both the source and destination link addresses to configure an OSPF sham link between two VPN PE routers.