Each link state advertisement has an LS age field. The LS age is
expressed in seconds. An advertisement's LS age field is
incremented while it is contained in a router's database. Also,
when copied into a Link State Update Packet for flooding out a
particular interface, the advertisement's LS age is incremented by
InfTransDelay.
An advertisement's LS age is never incremented past the value
MaxAge. Advertisements having age MaxAge are not used in the
routing table calculation. As a router ages its link state
database, an advertisement's LS age may reach MaxAge.[18] At this
time, the router must attempt to flush the advertisement from the
routing domain. This is done simply by reflooding the MaxAge
advertisement just as if it was a newly originated advertisement
(see Section 13.3).
When creating a Database summary list for a newly forming adjacency,
any MaxAge advertisements present in the link state database are
added to the neighbor's Link state retransmission list instead of
the neighbor's Database summary list. See Section 10.3 for more
details.
A MaxAge advertisement must be removed immediately from the router's
link state database as soon as both a) it is no longer contained on
any neighbor Link state retransmission lists and b) none of the
router's neighbors are in states Exchange or Loading.
When, in the process of aging the link state database, an
advertisement's LS age hits a multiple of CheckAge, its LS checksum
should be verified. If the LS checksum is incorrect, a program or
memory error has been detected, and at the very least the router
itself should be restarted.
