140 CHAPTER 6: IP ROUTING PROTOCOL OPERATION
Note the following items when you configure a STUB area:
■ The backbone area cannot be configured as a STUB area, and virtual links
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
the STUB area is 1.
Configuring the NSSA of OSPF
To keep the advantages of stub areas and simultaneously improve the networking
flexibility, RFC1587 (OSPF NSSA Option) defines a new type of area, namely NSSA,
which has the capability of importing external routes in a limited way.
An NSSA is similar to a Stub area. Neither of them generates or imports
AS-External-LSA (namely Type-5 LSA), and both of them can generate and import
Type-7 LSAs. Type-7 LSAs are generated by the ASBR of the NSSA area, which can
only advertise in the NSSA area. When a Type-7 LSA reaches the ABR of the NSSA,
the ABR decides whether to transform the Type-7 LSA into an AS-External-LSA so
as to advertise it to other areas.
For example, in Figure 32, the AS running OSPF comprises three areas: Area 1,
Area 2 and Area 0. Area 0 is the backbone area. There are another two ASs
running RIP. Area 1 is defined as an NSSA. After the RIP routes of the Area 1 are
propagated to the NSSA, ASBR generates type-7 LSAs which are propagated in
Area 1. When a type-7 LSA reaches the NSSA ABR, the NSSA ABR transforms it
into a type-5 LSA, which is propagated to Area 0 and Area 2.
RIP routes of the AS running RIP are translated into type-5 LSAs that are
propagated in the OSPF AS. However, type-5 LSAs do not reach Area 1 because
Area 1 is an NSSA. NSSA and STUB areas use the same policy in this respect.
The NSSA cannot be configured with virtual links.
Table 144 Configuring and OSPF STUB area
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
To Next Page
Loading...
