Foundry Configuration Guide for the FESX, FSX, and FWSX
20 - 2 © Foundry Networks, Inc. December 2005
• Not-So-Stubby Area (NSSA) external link
OSPF is built upon a hierarchy of network components. The highest level of the hierarchy is the Autonomous
System (AS). An autonomous system is defined as a number of networks, all of which share the same routing
and administration characteristics.
An AS can be divided into multiple areas as shown in Figure 20.1 on page 20-3. Each area represents a
collection of contiguous networks and hosts. Areas limit the area to which link-state advertisements are
broadcast, thereby limiting the amount of flooding that occurs within the network. An area is represented in OSPF
by either an IP address or a number.
You can further limit the broadcast area of flooding by defining an area range. The area range allows you to
assign an aggregate value to a range of IP addresses. This aggregate value becomes the address that is
advertised instead all of the individual addresses it represents being advertised. You can assign up to 32 ranges
in an OSPF area.
An OSPF router can be a member of multiple areas. Routers with membership in multiple areas are known as
Area Border Routers (ABRs). Each ABR maintains a separate topological database for each area the router is
in. Each topological database contains all of the LSA databases for each router within a given area. The routers
within the same area have identical topological databases. The ABR is responsible for forwarding routing
information or changes between its border areas.
An Autonomous System Boundary Router (ASBR) is a router that is running multiple protocols and serves as a
gateway to routers outside an area and those operating with different protocols. The ASBR is able to import and
translate different protocol routes into OSPF through a process known as redistribution. For more details on
redistribution and configuration examples, see “Enable Route Redistribution” on page 20-27.
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