3Com 4007, Implementation Manual, Page: 457 / 684

Key Concepts 457
Electing the Designated Router
OSPF selects a designated router, which originates LSAs on behalf of the
network segment. These advertisements list all routers (including the
designated router) that are attached to the segment. The designated
router also floods LSA packets throughout the segment to allow its
neighbors to update their databases.
The OSPF algorithm first eliminates all routers that have an assigned
priority of
0
. OSPF then selects a designated router from among the
routers that have declared themselves to be the designated router (based
on their configuration settings). If no routers have declared their
candidacy, the backup designated router becomes the designated router,
and OSPF selects a new backup designated router.
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.
The designated router then becomes adjacent to all other routers on the
network segment by sending Hello packets to them.
Calculating Shortest Path Trees
OSPF routers collect raw topological data from the LSAs that they receive.
Each router then prunes this data down to a tree of the shortest network
paths centered on itself. In a series of iterations, the router examines the
total cost to reach each router or network node in its domain. By
discarding all but the lowest-cost path to each destination, the router
builds a shortest path tree to each destination, which it uses until the
network topology changes.
Routing Packets
A packet’s source and destination determine the routers that move it:
■
Intraarea — When a packet’s source and destination are in the same
area, the packet is routed using internal router databases. No routers
are used outside the area.
■
Interarea — When a packet’s source and destination are in different
areas, the topology databases in the backbone area dictate the paths
that are taken between areas.
You can use virtual links to influence the routes that are taken for
interarea traffic. See “Virtual Links” later in this chapter.