Adjacency
In an OSPF routing network, when an OSPF router boots up it sends out OSPF Hello packets to find any neighbors, routers that have access to the same network as the router booting up. Once neighbors are discovered and Hello packets are exchanged, updates are sent, and the Link State databases of both neighbors are synchronized. At this point these neighbors are said to be adjacent.
For two OSPF routers to become neighbors, the following conditions must be met.
• The subnet mask used on both routers must be the same subnet.
• The subnet number derived using the subnet mask and each router's interface IP address must match.
• The Hello interval & The Dead interval must match.
• The routers must have the same OSPF area ID. If they are in different areas, they are not neighbors.
• If authentication is used, they must pass authentication checks.
If any of these parameters are different between the two routers, the routers do not become OSPF neighbors and cannot be adjacent. If the routers become neighbors, they are adjacent.
Adjacency and neighbors
Neighbor routers can be in a Two-Way state, and not be adjacent. Adjacent routers normally have a neighbour state of FULL. Neighbors only exchange Hello packets, and do not exchange routing updates. Adjacent routers exchange LSAs (LSDB information) as well as Hello packets. A good example of an adjacent pair of routers is the DR and BDR.
You can check on the state of an OSPF neighbor using the CLI command
get router info ospf neighbor all. See
“Checking the state of OSPF neighbors”.
Why adjacency is important
It is important to have adjacent pairs of routers in the OSPF routing domain because routing protocol packets are only passed between adjacent routers. This means adjacency is required for two OSPF routers to exchange routes. If there is no adjacency between two routers, such as one on the 172.20.120.0 network and another on the 10.11.101.0 network, the routers do not exchange routes. This makes sense because if all OSPF routers on the OSPF domain exchanged updates it would flood the network. Also, it is better for updates to progress through adjacent routers to ensure there are no outages along the way. Otherwise, updates could skip over routers that are potentially offline, causing longer routing outages and delays while the OSPF domain learns of this outage later on.
If the OSPF network has multiple border routers and multiple connections to external networks, the designated router (DR) determines which router pairs become adjacent. The DR can accomplish this because it maintains the complete topology of the OSPF domain, including which router pairs are adjacent. The BDR also has this information in case the DR goes offline.