Switch 7700 Configuration Guide

86 CHAPTER 5: ROUTING PROTOCOL OPERATION
By default, the STUB area is not configured, and the cost of the default route to a
STUB area is 1.
Configure NSSA of OSPF
NSSA and NSSA LSA (also called Type-7 LSA) are transformations of the STUB area
and are highly similar to a STUB area. NSSA does not allow importing
AS-External-LSAs (type-5 LSAs) but it does allow importing AS-External-LSA
(type-7 LSAs).
Type-7 LSAs are generated by the ASBRs in an NSSA, and propagated in the NSSA.
When the type-7 LSAs reach an ABR of the NSSA, the ABR translates the type-7
LSAs into AS-External-LSAs, which is propagated to the other areas.
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
Perform the following configuration in OSPF Area view.
Remove the cost of the default
route to the STUB area
undo default-cost
Tab le 35 Configure NSSA of OSPF
Operation Command
Configure an area to be the
NSSA area
nssa [ default-route-advertise ] [ no-import-route ] [
no-summary ]
Cancel the configured NSSA undo nssa
Configure the default cost
value of the route to the NSSA
default-cost cost
Table 34 Configure an OSPF STUB Area
Operation Command
RIP
RIP
Area 2
Area 0
Area 1
NSSA
NSSA
ABR
NSSA
ASBR