For adjacencies to form, OSPF must first have discovered its neighbors. Adjacencies are formed for the purpose of exchanging routing information. Not every neighboring router forms an adjacency. The conditions under which OSPF forms adjacencies are as follows:
• Network connectivity is point-to-point.
• Network connectivity is achieved through a virtual link.
• The neighboring router is the DR.
• The neighboring router is the BDR.
Adjacencies control the distribution of routing updates in the sense that only routers adjacent to the one sending an update process the update. Chapter 3, "OSPF Communication," discusses the forming of adjacencies in more detail.
This is a common question that should be explained easily yet is often not. Think of it this way. You live next to many different people, and you are considered neighbors to them as a result. You might wave at them when you drive to the store, but you are not close friends.
In certain circumstances, you become close friends with people with whom you were first neighbors. These close friends become known as adjacent; you talk to each other over the fence in the summer, have them over for dinner—that kind of thing. You essentially have a stronger level of communication with them.
You can determine which other routers you are adjacent with by using the show ip ospf interface command, as demonstrated in Example 2-5.
Example 2-5 Determining Router Adjacencies
HAL9000#show ip ospf interface
Ethernet0 is up, line protocol is up
Internet Address 192.168.254.253/24, Area 0
Process ID 100, Router ID 192.168.254.253, Network Type BROADCAST, Cost: 10 Transmit Delay is 1 sec, State DR, Priority 1
Designated Router (ID) 192.168.254.253, Interface address 192.168.254.253 Backup Designated router (ID) 192.168.254.102, Interface address 192.168.254.102 Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:06 Neighbor Count is 3, Adjacent neighbor count is 3
Adjacent with neighbor 192.168.254.102 (Backup Designated Router) Adjacent with neighbor 192.168.254.100 Adjacent with neighbor 192.168.254.101 Suppress hello for 0 neighbor(s) Serial0 is down, line protocol is down
OSPF not enabled on this interface HAL9000#
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