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. 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.