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456 C
HAPTER
19: O
PEN
S
HORTEST
P
ATH
F
IRST
(OSPF) R
OUTING
How OSPF Routing
Works
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
Starting Up
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.
Finding Neighbors
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.
Establishing Adjacencies
If neighboring OSPF routers succeed in exchanging and synchronizing
their link state databases, they appear as
adjacent in all router and
network link advertisements.
Electing the Backup Designated Router
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 that has the
highest router ID.