OSPF Sham Link’s
I came across the OSPF subcommand ‘Sham-link’ when studying for my CCNP Route exam, and was curious as to what it was.
The OSPF sham link provides a logical link between two VRFs. It creates a link that makes the MPLS PE’s participating in the sham link appear as a point to point link within OSPF. These links are able to fool or trick routers in the OSPF domain that this is a better path thus preserving the LSAs as type 1 or type 3.
When an OSPF sham-link is set it builds a bridge between two VRF’s. By advertising a type 1 LSA (Router) across this link, the OSPF database sees this route and the routes advertised across this link as acceptable. This is due to no longer being an external route and becoming an intra-area route.