3Com 8807 Configuration Manual page 327

Configuring NSSA Area
of OSPF
No ASBR can exist in a stub area. In other words, the external routes of the AS
■
cannot be propagated in the stub area.
Perform the following configuration in OSPF area view.
Table 299 Configure stub area of OSPF
Operation
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.
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.
Figure 82 NSSA area
RIP RIP
area 2 area 2
Perform the following configuration in OSPF area view.
NSSA NSSA
ABR ABR
area 1 area 1
NSSA NSSA
area 0 area 0
Configuring OSPF
Command
stub [ no-summary ]
undo stub
default-cost value
undo default-cost
NSSA NSSA
ASBR ASBR
RIP RIP
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