Understand Open Shortest Path First (ospf) Design Guide

Hello and Dead Intervals:
OSPF exchanges 
Hello 
packets on each segment. This is a form of
keepalive used by routers in order to acknowledge their existence on a segment and in order
to elect a designated router (DR) on multiaccess segments.
●
The 
Hello 
interval specifies the length of time, in seconds, between the 
Hello 
packets that a router
sends on an OSPF interface.
The dead interval is the number of seconds that a router 
Hello 
packets have not been seen before
its neighbors declare the OSPF router down.
OSPF requires these intervals to be exactly the same between two neighbors. If any of these
intervals are different, these routers do not become neighbors on a particular segment. The
router interface commands used to set these timers are: 
ip ospf hello-interval seconds
and 
ip ospf
dead-interval seconds
.
●
Stub area flag: 
Two routers have to also agree on the stub area flag in the 
Hello
packets in order
to become neighbors. Stub areas are discussed in a later section. Consider that definition of
stub areas affect the neighbor election process.
●
Adjacencies
Adjacency is the next step after the neighbor process. Adjacent routers are routers that go beyond
the simple 
Hello
exchange and proceed into the database exchange process.
In order to minimize the amount of information exchange on a particular segment, OSPF elects
one router to be a designated router (DR), and one router to be a backup designated router
(BDR), on each multi-access segment.
The BDR is elected as a backup mechanism in case the DR goes down. The idea behind this is
that routers have a central point of contact for information exchange.
Rather than exchange updates with every other router on the segment, every router exchanges
information with the DR and BDR.

---

The DR and BDR relay the information to everybo creationdy else. In mathematical terms, this
cuts the information exchange from O(n*n) to O(n) where n is the number of routers on a multi-
access segment.
This router model illustrates the DR and BDR:
In this diagram, all routers share a common multi-access segment. Due to the exchange of 
Hello
packets, one router is elected DR and another is elected BDR.
Each router on the segment (which already became a neighbor) tries to establish an adjacency
with the DR and BDR.