OSPF Hello timer
To discover neighbors and establish adjacency, OSPF routers exchange Hello packets at regular intervals called Hello timers. These Hello packets are sent to Multicast address group 18.104.22.168. Hello replies are unicast.
The Hello packet timer has some default values: in high speed links, the Hello timer is 5 sec. In Slow speed links, it is 60 sec (high speed links are ones that have greater or equal to the speed of a T1 circuit. And slow speed links are less than the speed of a T1 link.)
Neighbor routers can have different Hello timers, but their OSPF neighborship will be flapping.
The Hello protocol serves also as a keepalive mechanism between adjacent neighbors.
OSPF Hold-down timer
The Hold-down timer is usually equal to four times the value of the Hello timer:
Hold-down_timer = 4 * Hello_timer
OSPF dead interval
When router 1 no longer hears Hello packets from a neighbor router 2 for the duration of the Dead Interval, then it believes that router 2 has failed and re convergence calculations occur.