Information About OSPFv3 External Path Preference Option
The Open Shortest Path First version 3 (OSPFv3) external path preference option feature provides a way to calculate external path preferences per RFC 5340.
OSPFv3 External Path Preference Option
Per RFC 5340, the following rules indicate which paths are preferred when multiple intra-AS paths are available to ASBRs or forwarding addresses:
Intra-area paths using nonbackbone areas are always the most preferred.
The other paths, intraarea backbone paths and interarea paths, are of equal preference.
These rules apply when the same ASBR is reachable through multiple areas, or when trying to decide which of several AS-external-LSAs should be preferred. In the former case the paths all terminate at the same ASBR, and in the latter the paths terminate at separate ASBRs or forwarding addresses. In either case, each path is represented by a separate routing table entry. This feature applies only when RFC 1583 compatibility is set to disabled using the no compatibility rfc1583 command (RFC 5340 provides an update to RFC 1583).
To minimize the chance of routing loops, set identical RFC compatibility for all OSPF routers in an OSPF routing domain.