Neighbor discovery is the first step in getting a link state environment up and running. In keeping with the
friendly neighbor terminology, a Hello protocol is used for this step. The protocol will define a Hello
packet format and a procedure for exchanging the packets and processing the information the packets
contain.
NOTE
Router ID
At a minimum, the Hello packet will contain a router ID and the address of the network on which the
packet is being sent. The router ID is be something by which the router originating the packet can be
uniquely distinguished from all other routers; for instance, an IP address from one of the router's
interfaces. Other fields of the packet may carry a subnet mask, Hello intervals, a specified maximum
period the router will wait to hear a Hello before declaring the neighbor "dead," a descriptor of the circuit
type, and flags to help in bringing up adjacencies.
NOTE
Adjacent neighbors
When two routers have discovered each other as neighbors, they go through a process of synchronizing
their databases in which they exchange and verify database information until their databases are identical.
The details of database synchronization are described in Chapters 9, "Open Shortest Path First," and 10,
"Integrated IS-IS." To perform this database synchronization, the neighbors must be adja cent—that is,
they must agree on certain protocol-specific parameters such as timers and support of optional
capabilities. By using Hello packets to build adjacencies, link state protocols can exchange information in
a controlled fashion. Contrast this approach with distance vector, which simply broadcasts updates out
any interface configured for that routing protocol.
Beyond building adjacencies, Hello packets serve as keepalives to monitor the adjacency. If Hellos are
not heard from an adjacent neighbor within a certain established time, the neighbor is considered
unreachable and the adjacency is broken. A typical interval for the exchange of hello packets is 10
seconds, and a typical dead period is four times that.