Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 127
Maintaining the Database

After the algorithm is run, the databases are continually checked for synchronization between adjacencies using LSAs and the flooding procedure. The flooding procedure is simple: Receive an LSA, check for the information in the database, and determine whether or not to forward it to another adjacency using a LSA. To ensure reliability, the flooding procedure uses an acknowledgment procedure.

Reliability is also built into the protocol. When an LSA is transmitted, it is acknowledged. An unacknowledged packet is retransmitted by the issuing router until it is acknowledged.

Every LSA contains an age field. This field is used to age old entries in the database. If an entry is aged out, this information is flooded throughout the domain (a single area) and the Dykstra algorithm is run again to build a new router table.

Sequence numbers are generated for all LSAs. When a router transmits an LSA, it applies a sequence number to it. In this way, the receiving router will know if it is receiving the most recent information from anotherrouter. The sequence number is 32 bits long and is assigned to an LSA in ascending order.

Changes in the LSA database require a rerunning of the SPF algorithm and an update of the routing table depending on the outcome of the algorithm.