Designated router

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When an Open Shortest Path First (OSPF) router is first powered on, it must initialize itself. The only information it has to perform this task is the information it was configured with: its IP addresses and subnet masks, the link cost(s) for each of its links, and a variety of operating parameters. To begin operating quickly, it must get a copy of the current link state database from a neighbor.

A designated router furnishes a copy of the link state database to each new router in the network. An OSPF router identifies its neighbors using a special OSPF packet called a Hello packet. The designated router responds to the Hello packet with an acknowledgment. The new router then sends a Database Description packet, requesting a copy of the link state database. The designated router responds with another Database Description packet, furnishing the requested copy of the link state database.

The designated router also has other responsibilities. It is responsible for generating link state advertisements on behalf of the network(s) for which it is the designated router. It is also the task of the designated router to monitor the health of neighboring routers by periodically exchanging Hello packets with them.

Finally, the designated router is responsible for establishing adjacencies. Adjacencies are a way of establishing lines of communication between OSPF routers, and limiting the effects of flooded information. Two routers are neighbors if they share a common data link. Thus all of the routers attached to the same LAN are neighbors. However, routers form adjacencies with the only designated router and its backup.