| By configuring an area as an NSSA, you
can minimize routing tables 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 flooding
throughout a routing domain. When you define an NSSA, you can import
specific external routes as Type 7 LSAs into the NSSA. In addition,
when translating Type 7 LSAs to be imported into nonstub areas, you
can summarize or filter the LSAs 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. You can 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, you can extend
OSPF to handle the remote connection by defining the area between
the corporate router and the remote router as an NSSA.
In Figure ,
the central site and branch office are interconnected through a slow
WAN link. The branch office is not using OSPF, but the central site
is. If you configure a standard OSPF area between the two networks,
the slow WAN link could be overwhelmed by the ensuing flood of LSAs,
especially Type 5 external LSAs. As an alternative, you could
configure a RIP domain between the two networks, but that would mean
running two routing protocols on the central site's routers. A more
attractive 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/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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