3Com Switch 7700 Configuration Manual page 123

By default, the STUB area is not configured, and the cost of the default route to a
STUB area is 1.
Configuring NSSA of OSPF
An NSSA is similar to a STUB area. However, NSSA does not allow importing
AS-External-LSAs (type-5 LSAs) although it does allow importing
NSSA-External-LSAs (type-7 LSAs). ASBRs can be configured to convert type-5
LSAs to type-7 LSAs to allow advertising of type-5 LSAs within the NSSA. Similarly,
ABRs can be configured to reconvert the type-7 LSAs to type-5 LSAs as these LSAs
leave the NSSA.
For example, in Figure 5, the AS running OSPF includes three areas: Area 1, Area 2
and Area 0. Among them, Area 0 is the backbone area. Also, there are other two
ASs running RIP. Area 1 is defined as an NSSA. After RIP routes of Area 1 are
propagated to the NSSA ASBR, the NSSA ASBR generates type-7 LSAs which are
propagated in Area 1. When the type-7 LSAs reach the NSSA ABR, the NSSA ABR
translates it into a type-5 LSA, which is propagated to Area 0 and Area 2. On the
other hand, RIP routes of the AS running RIP are translated into type-5 LSAs that
are propagated in the OSPF AS. However, the type-5 LSAs do not reach Area 1
because Area 1 is an NSSA. NSSAs and STUB areas have the same approach in this
aspect.
Similar to a STUB area, the NSSA cannot be configured with virtual links.
Figure 5 NSSA
RIP
Area 2
Perform the following configuration in OSPF Area view.
Table 37 Configuring NSSA of OSPF
Operation
Configure an area to be the NSSA area
Cancel the configured NSSA
Configure the default cost value of the route
to the NSSA
Restore the default cost value of the route to
the NSSA area
All routers connected to the NSSA must use the nssa command to configure the
area with the NSSA attribute.
The default-route-advertise parameter is used to generate the default type-7
LSAs. The default type-7 LSA route is generated on the ABR, even though the
default route 0.0.0.0 is not in the routing table. On an ASBR, however, the default
NSSA
ABR
Area 1
NSSA
Area 0
Command
nssa [ default-route-advertise ] [
no-import-route ] [ no-summary ]
undo nssa
default-cost cost
undo default-cost
OSPF
NSSA
ASBR
RIP
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