OSPF Backdoor Links

OSPF backdoor links typically serve as backup paths, providing a way for traffic to flow from one VPN site to the other only if the path over the backbone is broken.

However, when the OSPF backdoor link connects two sites that are in the same OSPF area, the undesired result is that the path over the OSPF backdoor link is always preferred over the path over the backbone.

In Figure 112, the OSPF backdoor link connects customer site 4 to customer site 5 directly, without going through the backbone. OSPF uses the backdoor path for traffic flow between these two sites for the following reasons:

• At CE 4 and CE 5, the path over the OSPF backdoor link is an intra-area path, whereas the path over the backbone is an interarea path. OSPF always uses intra-area paths before interarea paths.
• At PE 2 and PE 3, the OSPF routes received from the respective directly connected CE routers have a better administrative distance than the IBGP routes received from the remote PE router. OSPF uses routes with better administrative distances.

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