Foundry Configuration Guide for the FESX, FSX, and FWSX
20 - 12 © Foundry Networks, Inc. December 2005
Assign a Not-So-Stubby Area (NSSA)
The OSPF Not So Stubby Area (NSSA) feature enables you to configure OSPF areas that provide the benefits of
stub areas, but that also are capable of importing external route information. OSPF does not flood external routes
from other areas into an NSSA, but does translate and flood route information from the NSSA into other areas
such as the backbone.
NSSAs are especially useful when you want to summarize Type-5 External LSAs (external routes) before
forwarding them into an OSPF area. The OSPF specification (RFC 2328) prohibits summarization of Type-5 LSAs
and requires OSPF to flood Type-5 LSAs throughout a routing domain. When you configure an NSSA, you can
specify an address range for aggregating the external routes that the NSSA's ABR exports into other areas.
The Foundry implementation of NSSA is based on RFC 1587.
Figure 20.5 shows an example of an OSPF network containing an NSSA.
Figure 20.5 OSPF network containing an NSSA
This example shows two routing domains, a RIP domain and an OSPF domain. The ASBR inside the NSSA
imports external routes from RIP into the NSSA as Type-7 LSAs, which the ASBR floods throughout the NSSA.
The ABR translates the Type-7 LSAs into Type-5 LSAs. If an area range is configured for the NSSA, the ABR also
summarizes the LSAs into an aggregate LSA before flooding the Type-5 LSA(s) into the backbone.
Since the NSSA is partially “stubby” the ABR does not flood external LSAs from the backbone into the NSSA. To
provide access to the rest of the Autonomous System (AS), the ABR generates a default Type-7 LSA into the
NSSA.
Configuring an NSSA
To configure OSPF area 1.1.1.1 as an NSSA, enter the following commands.
OSPF ABR
RIP Domain
NSSA Area 1.1.1.1
OSPF Area 0
Backbone
Internal ASBR
Foundry Router
Foundry Router Foundry Router
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