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13.1. Determining which link state is newer Connected: An Internet Encyclopedia
13.1. Determining which link state is newer

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Next: 13.2. Installing link state advertisements in the database

13.1. Determining which link state is newer

13.1. Determining which link state is newer

When a router encounters two instances of a link state advertisement, it must determine which is more recent. This occurred above when comparing a received advertisement to its database copy. This comparison must also be done during the Database Exchange procedure which occurs during adjacency bring-up.

A link state advertisement is identified by its LS type, Link State ID and Advertising Router. For two instances of the same advertisement, the LS sequence number, LS age, and LS checksum fields are used to determine which instance is more recent:

  • The advertisement having the newer LS sequence number is more recent. See Section 12.1.6 for an explanation of the LS sequence number space. 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.


Next: 13.2. Installing link state advertisements in the database

Connected: An Internet Encyclopedia
13.1. Determining which link state is newer

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