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
• Area ID number
• Subnet mask (for multiaccess networks)
• Stub area flag (described earlier)
• Authentication type and password

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.