What is the use of sham-link in OSPF?
For the configuration of MPLS VPN, OSPF usually acts as intermediate routing protocol of PE-CE to connect all places of VPN through MPLS backbone network. Although the connection of OSPF PE-CE ensures the continuity of VPN sites, backdoor link of VPN places should be considered. If two places belong to the same places, the path passing backdoor link is chosen at first. According to the rule of OSFP protocol, intra-area path is better than inter-area path. Control the route through policy if there is backdoor link.
If backdoor link just serves as backup and does not join VPN service, default handling process cannot be received. In order to build connection of places through MPLS VPN backbone area, logical intra-area link between ingress of PE router and egress VRF must be established. The feature offers one solution. Establish sham link between two places: sham-link, as intra-area channel, the communication of two places passes MPLS backbone ares and backdoor link acts as backup. If there is no backdoor link between two places, there is no need to use sham-link.