Cisco Nexus 7000 Series - Stub Area; Not-So-Stubby Area; Virtual Links

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6-9
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
OL-20002-02
Chapter 6 Configuring OSPFv2
Information About OSPFv2
Figure 6-3 shows an example of an OSPFv2 autonomous system where all routers in area 0.0.0.10 have
to go through the ABR to reach external autonomous systems. area 0.0.0.10 can be configured as a stub
area.
Figure 6-3 Stub Area
Stub areas use a default route for all traffic that needs to go through the backbone area to the external
autonomous system. The default route is 0.0.0.0 for IPv4.
Not-So-Stubby Area
A Not-so-Stubby Area (NSSA) is similar to a stub area, except that an NSSA allows you to import
autonomous system external routes within an NSSA using redistribution. The NSSA ASBR redistributes
these routes and generates NSSA External (type 7) LSAs that it floods throughout the NSSA. You can
optionally configure the ABR that connects the NSSA to other areas to translate this NSSA External LSA
to AS External (type 5) LSAs. The ABR then floods these AS External LSAs throughout the OSPFv2
autonomous system. Summarization and filtering are supported during the translation. See the
“Link-State Advertisements” section on page 6-5 for details on NSSA External LSAs.
You can, for example, use NSSA to simplify administration if you are connecting a central site using
OSPFv2 to a remote site that is using a different routing protocol. Before NSSA, the connection between
the corporate site border router and a remote router could not be run as an OSPFv2 stub area because
routes for the remote site could not be redistributed into a stub area. With NSSA, you can extend OSPFv2
to cover the remote connection by defining the area between the corporate router and remote router as
an NSSA (see the “Configuring NSSA” section on page 6-27).
The backbone Area 0 cannot be an NSSA.
Virtual Links
Virtual links allow you to connect an OSPFv2 area ABR to a backbone area ABR when a direct physical
connection is not available. Figure 6-4 shows a virtual link that connects Area 3 to the backbone area
through Area 5.
ABR
ASBR
Backbone Area 10
182984
Stub area