In order for OSPF routers exchange their routing databases; they must be able to discover themselves in the network.  For this discovery phase, OSPF-enabled routers use the Hello Protocol.

Hello Packet

The hello protocol is the responsible for establishing and maintaining an adjacency.

Hello packets ensure bi-directional communications between neighbors.

OSPF Hello packets are also used as a keepalive mechanism.   If the router does not hear the neighbor Hello packets in a given time (DeadInterval), it considers it down.

In Broadcast and Nonbroadcast environments, Hello packets are used to elect the Designated and Backup Designated Routers.   Another key point to remember is that on Broadcast networks, the destination address of the Hello packets is the multicast address 224.0.0.5.  In Nonbroadcast networks, the destination address is unicast.

The hello packets contain parameters that must match for routers to become neighbors:

1. The routers must be on a common subnet.
2. The Hello and Dead Interval timers must be identical between the neighbors.
3. The Area ID.
4. Options (Specifically the E-bit which denote the type of area. 1=normal, 0=stub).
5. The type and authentication password.

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