Understanding OSPF Sham Links

Figure 115 shows how you can use OSPF sham links to avoid the problem created by the intra-area backdoor link. The sham link is a logical intra-area link between VRF B on PE 2 and PE 3. OSPF creates an adjacency and exchanges LSAs across the sham link. As a result, OSPF sees both the path over the backdoor link and the path over the backbone as intra-area paths. OSPF then selects the best path based on the metrics of the links and selects the sham link path, ensuring that the backdoor link is not used.

Note: If the VPN sites are not connected by an OSPF backdoor link or if the VPN sites are in different OSPF areas, the problem does not exist and you do not need to configure an OSPF sham link.

Figure 115: OSPF Sham Link

OSPF Sham Link

Use the remote-neighbor command to configure the OSPF sham link on both VRFs joined by the link. If a BGP route and an OSPF route to the same destination are both installed in the IP routing table, OSPF uses the OSPF route because it has a better administrative distance by definition.

If you redistribute OSPF routes into BGP in each VRF, you do not want the OSPF routes that point to sham links to be redistributed into BGP. If they were redistributed, multiple BGP routes for a single OSPF route would exist: one BGP route at each endpoint of a sham link.

Use the dont-install-routes command to prevent OSPF routes pointing to the sham link from being installed in the IP routing table of the VRF, and thus to prevent them from being redistributed into BGP. Forwarding still works using the MP-IBGP routes received from the remote PE router.

Using this command avoids having many BGP routes to the same prefix by preventing OSPF routes learned over the sham link from being redistributed back into BGP even when you have configured redistribution of OSPF routes into BGP.

Use the ttl command to configure a TTL for the remote neighbor because the neighbor might be more than a single hop away. Use the update-source command to specify the loopback address used as the source address for the OSPF connection to the remote neighbor.

If you do not configure a sham link between each pair of PE routers for which a backdoor link exists, then you need to redistribute BGP routes back into OSPF.

For more information about OSPF remote neighbors, see Remote Neighbors in the JunosE IP, IPv6, and IGP Configuration Guide.

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