The sequence number field is a signed 32-bit integer. It is used to detect old and duplicate link state advertisements. The space of sequence numbers is linearly ordered. The larger the sequence number (when compared as signed 32-bit integers) the more recent the advertisement. To describe to sequence number space more precisely, let N refer in the discussion below to the constant 2**31.
The sequence number -N (0x80000000) is reserved (and unused). This leaves -N + 1 (0x80000001) as the smallest (and therefore oldest) sequence number. A router uses this sequence number the first time it originates any link state advertisement. Afterwards, the advertisement's sequence number is incremented each time the router originates a new instance of the advertisement. When an attempt is made to increment the sequence number past the maximum value of N - 1 (0x7fffffff), the current instance of the advertisement must first be flushed from the routing domain. This is done by prematurely aging the advertisement (see Section 14.1) and reflooding it. As soon as this flood has been acknowledged by all adjacent neighbors, a new instance can be originated with sequence number of -N + 1 (0x80000001).
The router may be forced to promote the sequence number of one of its advertisements when a more recent instance of the advertisement is unexpectedly received during the flooding process. This should be a rare event. This may indicate that an out-of-date advertisement, originated by the router itself before its last restart/reload, still exists in the Autonomous System. For more information see Section 13.4.