OSPF Determining which LSA is newer

When populating the LSDB, OSPF must determine if a received LSA is more recent than the information it already has. It will consider the LSA to be more recent if it has:

  • The LSA having the newer LS sequence number is more recent. If both instances have the same LS sequence number, then:
  • If the two instances have different LS checksums, then the instance having the larger LS checksum (when considered as a 16-bit unsigned integer) is considered more recent.
  • Else, if only one of the instances has its LS age field set to MaxAge, the instance of age MaxAge is considered to be more recent.
  • Else, if the LS age fields of the two instances differ by more than MaxAgeDiff, the instance having the smaller (younger) LS age is considered to be more recent.

Else, the two instances are considered to be identical.

Links: https://forum.networklessons.com/t/ospf-lsas-and-lsdb-flooding-tutorial/915/70?u=lagapides https://datatracker.ietf.org/doc/html/rfc2328#page-146

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