OSPF Convergence

The OSPF routing protocol converges within seconds because neighbor
routers only exchange “Hello” messages initially. During the convergence
process, routers get to know each other, exchanging communication param-eters and setting up their  neighbor table.
Routers can only become neighbors if the following things occur:
 ✦ They have successfully exchanged “Hello” messages
 ✦ They have interfaces in the same routing domain (that is, in the same
autonomous system)
 ✦ They have their hello timers set to the same values. Hello timers define
the following:
  •  The frequency at which routers send each “Hello” message to each
other
  •  How long neighbors wait before they consider a router out of network
Hello messages are not only sent during the convergence process but also
afterward to keep track of which routers are still in the network. If a router
becomes unavailable and stops sending Hello messages, one of the hello
timers defines how long neighbor routers wait for a Hello message before
they consider the unavailable router out of the network