Why configure sham links for OSPF on S series switches

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 intra-area link between VPN sites should also be considered. For two sites that belong to the same site, the path through the intra-area link is always preferred because, according to OSPF specifications, an intra-area path is always preferred to an inter-area path. Therefore, when an intra-area link exists, you can control routes through a policy.

If the intra-area link is used for backup only, 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. An OSPF sham link can be established between two sites as an intra-area channel to enable the two sites to communicate with each other through the MPLS backbone area, and the intra-area link is used for backup. If no intra-area link exists between the two sites, no sham link is required.

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