Switch 7700 Configuration Guide, v2

86 CHAPTER 5: ROUTING PROTOCOL OPERATION
To insure that routes to the destinations outside the AS are still reachable, the ABR
in this area generates a default route (0.0.0.0) and advertises it to the non-ABR
routers in the area.
Note the following items when you configure a STUB area:
■ The backbone area cannot be configured as a STUB area and the virtual link
cannot pass through the STUB area.
■ If you want to configure an area as a STUB area, all the routers in this area
should be configured with the stub command.
■ No ASBR can exist in a STUB area and the external routes of the AS cannot be
propagated in the STUB area.
Perform the following configuration in OSPF Area view.
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.
Tab le 35 Configure an OSPF STUB Area
Operation Command
Configure an area as the STUB area stub [no-summary]
Remove the configured STUB area undo stub
Set the cost of the default route to
the STUB area
default-cost value
Remove the cost of the default
route to the STUB area
undo default-cost