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

Chapter 126
Adjacency

After the Hello discovery process has allowed for the DR and BDR to be selected, routers on a single LAN segment determine whether to form an adjacency with one another. An adjacency is important because it enables two routers to allow the exchange of routing information through link-state advertisements. The following are the requirements for establishing an adjacency:

•  The link is a point-to-point link or a virtual link (discussed later).

•  The router is the DR or BDR.

•  The neighbor is the DR or BDR.

So, you can see that if the router is the DR or BDR, an adjacency is formed between the DR/BDR and an attached router. If these conditions are not met, then an adjacency is not formed. That is, not all routers form adjacencies with each other, only with the DR and BDR or a point-to-point link.

As the adjacency is formed, the “adjacent” routers’ databases must become “synchronized.” That is, each must contain the exact same information. There is a series of steps before full adjacency. The reason for this is to synchronize the link-state database. The adjacent routers transmit to adjacent neighbors a summary list of LSAs using the database description packet. The router takes this information, compares it to its own LSA database, and then builds a request list of LSAs that are in the received summary list but not in its LSA database, and LSAs that are in the database but not in the received information from its adjacent neighbor.

Adjacency

This newly build request list is then transmitted to its neighbor using the Link State request packet. Each router that receives this request list responds to each requested record in the list. The router that received the request packet responds with a Link State Update packet. Neighbors are considered to be fully adjacent when they have received all responses to the requests and become fully adjacent on a one-on-one basis with each router that has formed an adjacency. After the routers become fully adjacent, each will run the SPF algorithm using the information supplied in the database. The outcome of the algorithm is OSPF routes, which are added to the routing table.