Installing and Administering Internet Services

256 Chapter 8
Configuring gated
Configuring the OSPF Protocol
Configuring the OSPF Protocol
OSPF is a link-state routing protocol designed to distribute routing
information between routers in a single autonomous system (AS). Each
OSPF router transmits a packet with a description of its local links to all
other OSPF routers. The distributed database is built from the collected
descriptions. Using the database information, each router constructs its
own routing table of shortest paths from itself to each destination in the
AS.
OSPF allows routers, networks, and subnetworks within an AS to be
organized into subsets called areas. An area is a grouping of logically
contiguous networks and hosts. Instead of maintaining a topological
database of the entire AS, routers in an area maintain the topology only
for the area in which they reside. Therefore, all routers that belong to an
area must be consistent in their configuration of the area. The topology of
an area is hidden from systems that are not part of the area. The
creation of separate areas can help minimize overall routing traffic in the
AS. Figure 8-3 shows an example of three separate areas defined for an
AS.
Figure 8-3 Areas Defined in an Autonomous System
1
AB
23
F
Area 1
CD
E
Other AS
I
6
H
5G
4
Area 2 Area 3
Legend:
Network
Router
Area
Backbone