Neighbors

OSPF considers two routers that have an interface located on a common network as neighbors. When OSPF discovers its neighbors, this is the first step of discovering the network and building a routing table. This process begins with the router learning the router identification numbers of its neighbors via multicast Hello packets.

Hello packets are transmitted every 10 seconds for broadcast and point-to-point interfaces and every 30 seconds on NBMA interfaces. Hello packets are sent to a multicast destination address of 224.0.0.5 (AllSPFRouters); this allows all routers running OSPF to receive and process the Hello packets.

A neighbor relationship begins when the routers exchanging Hello packets see their own RID in the other router's Hello packet and they agree upon the following:

• Hello/dead transmission intervals

• Subnet mask (for multiaccess networks)

82 Chapter 2: Introduction to OSPF

• Stub area flag (described earlier)

• Authentication type and password

The output in Example 2-4 helps you determine the neighbor relationships that your router has formed. In this example, the router HAL9000 has a relationship with all the other routers to which it is attached.

Example 2-4 Determining Router OSPF Neighbor Relationships

HAL9000#show ip

ospf

neighbor

Neighbor ID

Pri

State

Dead Time

Address

Interface

192.168.254.102

1

FULL/BDR

00:00:37

192.168

254

102

Ethernet0

192.168.254.100

1

FULL/DROTHER

00:00:32

192.168

254

100

Ethernet0

192.168.254.101

1

FULL/DROTHER

00:00:34

192.168

254

101

Ethernet0

HAL9000#

TIP To build stable OSPF neighbor relationships, ensure that the number of routers per LAN is small. Use the priority command to organize which is the DR, and avoid having the same router as the DR for more than one link through the use of the ip ospf priority command.

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