The LSA format should include a field for the age of the advertisement. When an LSA is created, the
router sets this field to zero. As the packet is flooded, each router increments the age of the
advertisement.
Maximum age difference
This process of aging adds another layer of reliability to the flooding process. The protocol defines a
maximum age difference (MaxAgeDiff) value for the internetwork. A router may receive multiple copies
of the same LSA with identical sequence numbers but different ages. If the difference in the ages is lower
than the MaxAgeDiff, it is assumed that the age difference was the result of normal network latencies; the
original LSA in the database is retained, and the newer LSA (with the greater age) is not flooded. If the
difference is greater than the MaxAgeDiff value, it is assumed that an anomaly has occurred in the
internetwork in which a new LSA was sent without incrementing the sequence number. In this case, the
newer LSA will be recorded, and the packet will be flooded. A typical MaxAgeDiff value is 15 minutes
(used by OSPF).
NOTE
Maximum age
The age of an LSA continues to be incremented as it resides in a link state database. If the age for a link
state record is incremented up to some maximum age (MaxAge)—again defined by the specific routing
protocol—the LSA, with age field set to the MaxAge value, is flooded to all neighbors and the record is
deleted from the databases.
NOTE
Link state refresh time
If the LSA is to be flushed from all databases when MaxAge is reached, there must be a mechanism to
periodically validate the LSA and reset its timer before MaxAge is reached. A link state refresh time
(LSRefeshTime)[11] is established; when this time expires, a router floods a new LSA to all its neighbors,
who will reset the age of the sending router's records to the new received age. OSPF defines a MaxAge of
1 hour and an LSRefreshTime of 30 minutes. 100