7.2. The Synchronization of Databases

In a link-state routing algorithm, it is very important for all routers' topological databases to stay synchronized. OSPF simplifies this by requiring only adjacent routers to remain synchronized. The synchronization process begins as soon as the routers attempt to bring up the adjacency. Each router describes its database by sending a sequence of Database Description packets to its neighbor. Each Database Description Packet describes a set of link state advertisements belonging to the router's database. When the neighbor sees a link state advertisement that is more recent than its own database copy, it makes a note that this newer advertisement should be requested.

This sending and receiving of Database Description packets is called the "Database Exchange Process". During this process, the two routers form a master/slave relationship. Each Database Description Packet has a sequence number. Database Description Packets sent by the master (polls) are acknowledged by the slave through echoing of the sequence number. Both polls and their responses contain summaries of link state data. The master is the only one allowed to retransmit Database Description Packets. It does so only at fixed intervals, the length of which is the configured constant RxmtInterval.

Each Database Description contains an indication that there are more packets to follow --- the M-bit. The Database Exchange Process is over when a router has received and sent Database Description Packets with the M-bit off.

During and after the Database Exchange Process, each router has a list of those link state advertisements for which the neighbor has more up-to-date instances. These advertisements are requested in Link State Request Packets. Link State Request packets that are not satisfied are retransmitted at fixed intervals of time RxmtInterval. When the Database Description Process has completed and all Link State Requests have been satisfied, the databases are deemed synchronized and the routers are marked fully adjacent. At this time the adjacency is fully functional and is advertised in the two routers' link state advertisements.

The adjacency is used by the flooding procedure as soon as the Database Exchange Process begins. This simplifies database synchronization, and guarantees that it finishes in a predictable period of time.