MPLS Layer 3 VPN PE-CE OSPF Sham Link

Hello Pradyumna

Whenever you require a sham link for OSPF, this is due to the fact that a customer has obtained a backup link between the customer sites, most often with the goal of providing redundancy. This means that there will be two possible paths that data can take between two particular customer sites, one via the MPLS network, and the other via the backup link.

Usually, an MPLS network will be the primary method of interconnecting sites, due to the fact that it is typically a high performance network, and because customers will often have more than two sites. But some customers may want to use MPLS as backup. So the sham link is used only when you want the MPLS network to be the primary link that OSPF routing chooses.

You are correct, there are indeed two links between customer sites in a topology such as the one that is described in this lesson, one via MPLS and one via the backup link.

I hope this has been helpful!

Laz