TCP/IP Tutorial and Technical Overview - IBM Redbooks

5.6.2 Neighbor communication
OSPF is responsible for determining the optimum set of paths through a network.
To accomplish this, each router exchanges LSAs with other routers in the
network. The OSPF protocol defines a number of activities to accomplish this
information exchange:
► Discovering neighbors
► Electing a designated router
► Establishing adjacencies and synchronizing databases
The five OSPF packet types are used to support these information exchanges.
Discovering neighbors: The OSPF Hello protocol
The Hello protocol discovers and maintains relationships with neighboring
routers. Hello packets are periodically sent out to each router interface. The
packet contains the RID of other routers whose hello packets have already been
received over the interface.
When a device sees its own RID in the hello packet generated by another router,
these devices establish a neighbor relationship.
The hello packet also contains the router priority, DR identifier, and BDR
identifier. These parameters are used to elect the DR on multi-access networks.
Electing a designated router
All multi-access networks must have a DR. A BDR can also be selected. The
backup ensures there is no extended loss of routing capability if the DR fails.
The DR and BDR are selected using information contained in hello packets. The
device with the highest OSPF router priority on a segment becomes the DR for
that segment. The same process is repeated to select the BDR. In case of a tie,
the router with the highest RID is selected. A router declared the DR is ineligible
to become the BDR.
After elected, the DR and BDR proceed to establish adjacencies with all routers
on the multi-access segment.
Establishing adjacencies and synchronizing databases
Neighboring routers are considered adjacent when they have synchronized their
link state databases. A router does not develop an adjacency with every
neighboring device. On multi-access networks, adjacencies are formed only with
the DR and BDR. This is a two step process.
Chapter 5. Routing protocols 205