7.1. The Hello Protocol

The Hello Protocol is responsible for establishing and maintaining neighbor relationships. It also ensures that communication between neighbors is bidirectional. Hello packets are sent periodically out all router interfaces. Bidirectional communication is indicated when the router sees itself listed in the neighbor's Hello Packet.

On multi-access networks, the Hello Protocol elects a Designated Router for the network. Among other things, the Designated Router controls what adjacencies will be formed over the network (see below).

The Hello Protocol works differently on broadcast networks, as compared to non-broadcast networks. On broadcast networks, each router advertises itself by periodically multicasting Hello Packets. This allows neighbors to be discovered dynamically. These Hello Packets contain the router's view of the Designated Router's identity, and the list of routers whose Hello Packets have been seen recently.

On non-broadcast networks some configuration information is necessary for the operation of the Hello Protocol. Each router that may potentially become Designated Router has a list of all other routers attached to the network. A router, having Designated Router potential, sends Hello Packets to all other potential Designated Routers when its interface to the non- broadcast network first becomes operational. This is an attempt to find the Designated Router for the network. If the router itself is elected Designated Router, it begins sending Hello Packets to all other routers attached to the network.

After a neighbor has been discovered, bidirectional communication ensured, and (if on a multi-access network) a Designated Router elected, a decision is made regarding whether or not an adjacency should be formed with the neighbor (see Section 10.4). An attempt is always made to establish adjacencies over point-to-point networks and virtual links. The first step in bringing up an adjacency is to synchronize the neighbors' topological databases. This is covered in the next section.