3Com Switch 8800 Configuration Guide
Chapter 19 OSPF Configuration
19-25
Table 19-28
Configure stub area of OSPF
Operation
Command
Configure an area to be the stub area
stub
[
no-summary
]
Remove the configured stub area
undo
stub
Configure the cost of the default route transmitted by
OSPF to the stub area
default-cost
value
Remove the cost of the default route to the stub area
undo
default-cost
By default, the stub area is not configured, and the cost of the default route to the stub
area is 1.
19.2.22 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.
area 0
area 1
NSSA
area 2
RIP
RIP
NSSA
ASBR
NSSA
ABR
area 0
area 1
NSSA
area 2
RIP
RIP
NSSA
ASBR
NSSA
ABR
Figure 19-2
NSSA area