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By configuring an area as an NSSA, routing
tables can be minimized within the area but still import external
routing information into OSPF.
Figure
illustrates the example network, including an NSSA implementation. RTA
can import external routes as Type 7 LSAs, and ABRs will translate
Type 7 LSAs into Type 5 LSAs as they leave the NSSA. A benefit of Type
7 LSAs is that they can be summarized. The OSPF specification
prohibits the summarizing or filtering of Type 5 LSAs. It is an OSPF
requirement that Type 5 LSAs always be flooded throughout a routing
domain. When defining an NSSA, specific external routes can be
imported as Type 7 LSAs into the NSSA. In addition, when translating
Type 7 LSAs to be imported into nonstub areas, the LSAs can be
summarized or filtered before importing them as Type 5 LSAs.
NSSAs are often used when a remote
site, which uses RIP or IGRP, must be connected to a central site
using OSPF. Use NSSA to simplify the administration of this kind of
topology. Before NSSA, the connection between the corporate site ABR
and the remote router used RIP or EIGRP. This meant maintaining two
routing protocols. Now, with NSSA, OSPF can be extended to handle the
remote connection by defining the area between the corporate router
and the remote router as an NSSA.
Figure
shows the
central site and branch offices interconnected through a slow WAN
link. The branch office is not using OSPF, but the central site is. If
a standard OSPF area between the two networks is configured, the slow
WAN link could be overwhelmed by the ensuing flood of LSAs. This is
especially true for Type 5 external LSAs. As an alternative, configure
a RIP domain between the two networks, but that would mean running two
routing protocols on the central site routers. A better solution is
to configure an OSPF area and define it as a NSSA.
In this scenario, RTA is defined as an
ASBR. It is configured to redistribute any routes within the RIP or EIGRP
domain to the NSSA. The following is a description of what happens
when the area between the connecting routers is defined as an NSSA:
- RTA receives RIP or EIGRP routes for
networks 10.10.0.0/16, 10.11.0.0/16, and 20.0.0.0/8.
- Because RTA is also connected to an
NSSA, it redistributes the RIP or EIGRP routes as Type 7 LSAs into
the NSSA.
- RTB, an ABR between the NSSA and the
backbone Area 0, receives the Type 7 LSAs.
- After the SPF calculation on the
forwarding database, RTB translates the Type 7 LSAs into Type 5 LSAs
and then floods them throughout Area 0.
It is at this point that RTB could have
summarized routes 10.10.0.0/16 and 10.11.0.0/16 as 10.0.0.0/8, or
could have filtered one or more of the routes.
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