H3C S9500 Series Operation Manual page 268

Operation Manual – Routing Protocol
H3C S9500 Series Routing Switches
Table 4-31 Configure stub area of OSPF
Configure an area to be the stub area
Remove the configured stub area
Configure the cost of the default route transmitted by
OSPF to the stub area
Remove the cost of the default route to the stub area
By default, the stub area is not configured, and the cost of the default route to the stub
area is 1.
4.2.22 Configuring NSSA Area of OSPF
RFC1587 introduced a new type of area called NSSA area, and a new type of LSA
called NSSA LSA (or Type-7 LSA).
NSSA areas are virtually variations of Stub areas. They are similar in many ways.
Neither of them generates or imports AS-External-LSA (namely Type-5 LSA), and both
of them can generate and import Type-7 LSA. Type-7 LSA is generated by ASBR of
NSSA area, which can only be advertised in NSSA area. When Type-7 LSA reaches
ABR of NSSA, ABR will select whether to transform Type-7 LSA into AS-External-LSA
so as to advertise to other areas.
For example, in the network below, the AS running OSPF comprises three areas: Area
1, Area 2 and Area 0. Among them, Area 0 is the backbone area. Also, there are other
two ASs respectively running RIP. Area 1 is defined as an NSSA area. After RIP routes
of the Area 1 are propagated to the NSSA ASBR, the NSSA ASBR will generate type-7
LSAs which will be propagated in Area 1. When the type-7 LSAs reach the NSSA ABR,
the NSSA ABR will transform it into type-5 LSA, which will be propagated to Area 0 and
Area 2. On the other hand, RIP routes of the AS running RIP will be transformed into
type-5 LSAs that will be propagated in the OSPF AS. However, the type-5 LSAs will 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.
RIP RIP
Figure 4-2 NSSA area
Operation
area 0 area 0
area 2 area 2
NSSA NSSA
ABR ABR
area 1 area 1
NSSA NSSA
RIP RIP
4-26
Chapter 4 OSPF Configuration
Command
stub [ no-summary ]
undo stub
default-cost value
undo default-cost
NSSA NSSA
ASBR ASBR