
IP Concepts, Terminology, and Features
117356-E Rev 00
1-13
OSPF Neighbors
OSPF neighbors are any two routers that have an interface to the same network.
In each OSPF network, routers use the Hello protocol to discover their neighbors
and maintain neighbor relationships. On a broadcast or point-to-point network,
the Hello protocol dynamically discovers neighbors; however, on a nonbroadcast
multi-access network, you must manually configure neighbors.
The Hello protocol ensures that communication between neighbors is
bidirectional. Periodically, OSPF routers send out Hello packets over all
interfaces. Included in these Hello packets is the following information:
• The router’s priority
• The router’s Hello timer and dead timer values
• A list of routers that have sent Hello packets to this router on this interface
• The router’s choice for designated router and backup designated router
Bidirectional communication is verified when one router sees itself listed in the
neighbor’s Hello packet.
Neighbor Adjacencies
Neighbors may form an adjacency for the purpose of exchanging routing
information. When two routers form an adjacency, they go through a process
called database exchange to synchronize their topological databases. When their
databases are synchronized, the routers are said to be fully adjacent. From this
point on, only routing change information is passed between the routers, thus
conserving bandwidth.
Routers connected by a point-to-point network or a virtual link always form an
adjacency. Also, every router on a multiaccess network forms an adjacency
relationship with the designated router and the backup designated router.
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