Adjacency Requirements

Adjacency Requirements
Once neighbors have been identified, adjacencies must be established so that routing (LSA)
information can be exchanged. There are two steps required to change a neighboring OSPF
router into an adjacent OSPF router:
1.
Establish two-way communication (achieved via the Hello protocol).
2.
Establish database synchronization—this consists of three packet types being exchanged
between routers:

Database Description (DD) packets

Link-State Request (LSR) packets

Link-State Update (LSU) packets
Once the database synchronization has taken place, the two routers are considered adjacent.
This is how adjacency is achieved, but you must also know when an adjacency will occur.
When adjacencies form depends on the network type. If the link is point-to-point, the two neighbors
will become adjacent if the Hello packet information for both routers is configured properly.
On broadcast multi-access networks, adjacencies are formed only between the OSPF routers
on the network and the DR and BDR, as well as between the DR and BDR. Figure 5.2 illustrates
an example. Three types of routers are pictured: DR, BDR, and DROther. DROther routers are
routers that have interfaces on the same network as the DR and BDR but only represent their
own router links, not the network, via LSAs.