What is an OSPF sham link and what is the function of an OSPF sham link

In Multiprotocol Label Switching (MPLS) Virtual Private Network (VPN) configuration, OSPF is used as the routing protocol between PEs and CEs so that the sites in a VPN can be connected through the MPLS backbone network. Though the connectivity between OSPF PEs and CEs ensures the connectivity between VPN sites, the backdoor link between VPN sites should also be considered. For two sites to the same site, the path through the backdoor link is always preferred because, according to OSPF specifications, an intra-area path is always preferred to an inter-area path. Therefore, when a backdoor link exists, consider controlling routes through policies.

If the backdoor link is used for backup only but is not used to provide VPN services, the default processing flow will be unacceptable. For connections to be re-established between sites through the MPLS VPN backbone area, a logical intra-area link must be established between the ingress and egress Virtual Routing and Forwarding (VRF) interfaces of the related PEs. This feature provides a solution to establish an OSPF sham link between two sites as an intra-area channel so that the two sites communicate with each other through the MPLS backbone area, while the backdoor link is used for backup. If no backdoor link exists between the two sites, no sham link is needed.

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