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7-9
Cisco Nexus 7000 Series NX-OS Unicast Routing Configuration Guide, Release 4.x
OL-20002-02
Chapter 7 Configuring OSPFv3
Information About OSPFv3
Stub Area
You can limit the amount of external routing information that floods an area by making it a stub area. A
stub area is an area that does not allow AS External (type 5) LSAs (see the “Link-State Advertisement”
section on page 7-5). These LSAs are usually flooded throughout the local autonomous system to
propagate external route information. Stub areas have the following requirements:
• All routers in the stub area are stub routers. See the “Stub Routing” section on page 1-7.
• No ASBR routers exist in the stub area.
• You cannot configure virtual links in the stub area.
Figure 7-3 shows an example an OSPFv3 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 7-3 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 an Inter-Area-Prefix LSA with prefix length set to 0 for IPv6.
Not-So-Stubby Area
A Not-So-Stubby Area (NSSA) is similar to the 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 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 Type-7 LSA to AS External
(type 5) LSAs. The ABR then floods these AS External LSAs throughout the OSPFv3 autonomous
system. Summarization and filtering are supported during the translation. See the “Link-State
Advertisement” section on page 7-5 for details on Type-7 LSAs.
You can, for example, use NSSA to simplify administration if you are connecting a central site using
OSPFv3 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 OSPFv3 stub area because
routes for the remote site could not be redistributed into a stub area. You needed to maintain two routing
protocols. With NSSA, you can extend OSPFv3 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 7-23).
The backbone Area 0 cannot be an NSSA.
ABR
ASBR
Backbone Area 10
182984
Stub area
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