3Com CoreBuilder 3500, Implementation Manual

Page: 363 / 592

Key Concepts 363
Key Concepts
Before you configure OSPF on your system, review the following key
concepts and terms discussed in these sections:
■
Autonomous Systems
■
Areas
■
Neighbors and Adjacency
■
Router Types
■
Protocol Packets
■
How OSPF Routing Works
Autonomous Systems An autonomous system consists of a set of OSPF routers that exchange
routing information. The network shown in Figure 70 later in this chapter
contains two autonomous systems.
Using identical topology databases, each router in an autonomous system
calculates shortest-path routes from itself to every known destination in
the autonomous system. The routers create their topology databases
using the data in link state advertisements (LSAs) from other routers in
the autonomous system.
Areas Autonomous systems can be subdivided into smaller, more manageable,
groups of contiguous networks called
areas
. Each OSPF router in an area
must have identical topological link state databases. These databases may
include area links, summarized links, and external links that depict the
topology of the autonomous system.
Neighbors and
Adjacency
Instead of each router sending routing information to every other router
on the network, OSPF routers establish adjacencies among neighboring
routers. Only adjacent routers exchange routing information. This
information is exchanged using Database Description packets, which are
used to describe the contents of each router’s link state database.