How Ospf Routing Works; Starting Up; Finding Neighbors; Establishing Adjacencies - 3Com CoreBuilder 2500 User Manual

7-12 C 7: R
HAPTER OUTING WITH
How OSPF Routing
Works

Starting Up

Finding Neighbors

Establishing
Adjacencies
Electing the Backup
Designated Router
OSPF
This section summarizes how the OSPF algorithm works for a router that
meets these characteristics:
Lies within an autonomous system area (an interior router)
Is attached to a multiaccess network
Is configured to be the designated router for its network
When the router starts, the interfaces that are configured to run OSPF
begin in the down state. When the lower-level IP protocols indicate that
an interface is available, the interface moves to the waiting state. It
remains in this state until the designated router and backup designated
router are chosen.
The router sends out hello packets to locate its network neighbors. These
packets also list the routers from which the sending router has received
hello packets. When a router detects its own address in another router's
hello packet, the two routers establish two-way communications as
neighbors.
If neighboring OSPF routers succeed in exchanging and synchronizing
their link state databases, they appear as adjacent in all router and
network link advertisements.
OSPF selects a backup designated router for the network segment. This
router takes over as the designated router if the current designated router
fails.
The OSPF algorithm first eliminates all routers that have an assigned
priority of 0. OSPF then selects the backup designated router from among
the routers on the segment that have not declared themselves to be the
designated router (based on their configuration settings). If some routers
have already declared themselves to be the backup designated router,
OSPF limits the selection to one of them.
OSPF selects the candidate router with the highest priority. If candidate
routers have the same priority, OSPF selects the router with the highest
router ID.