Advertising Unreachable Links in OSPF

OSPF Router Link State Advertisements (LSAs) use fixed-format encodings that always include advertised links in the default SPF (Shortest Path First) computation. For non-default SPF computations, e.g., flexible algorithms as described in RFC 9350, advertised OSPF links are used in the default SPF computation even if this is not intended. In order to advertise these links and not use them in the base SPF calculation, the metric LSLinkInfinity (0xffff) is used to specify that the link is unreachable. If all OSPF routers in an OSPF area support this functionality and have advertised the capability via an area-scoped OSPF Router-Information LSA, then links advertised with a metric of LSLinkInfinity are considered unreachable. MaxReachableLinkMetric (0xfffe) is defined to provide backward compatible reachability in specifications that previously specified advertisement of MaxLinkMetric (0xffff). This document updates RFC 5443, RFC 6987, RFC 8379, and RFC 8770 with respect to the advertisement of MaxReachableLinkMetric (0xfffe) rather than MaxLinkMetric (0xffff).