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432 CHAPTER 29: CONFIGURING OSPF
information is reduced greatly. A default routing (0.0.0.0) is generated for the area
by the ABR of the area to insure that these routes are reachable. A stub area is an
optional configured attribute, but it does not mean that each area is configurable.
Usually, a stub area is located at the boundary of the AS. A non-backbone area
with only one ABR or multi-ABR that are not virtually connected between ABRs
can be configured as a stub area.
A “totally stubby area” is the area that does not receive Type-3, Type-4, and Type-5
LSA (excluding Type-3 LSA which contains default routing and is generated by the
ABR). Inside such areas, there is no route to the outside and other areas of the AS,
so the scale of the routing table and the quantity of the transmitted routing
information is less.
A totally stubby area is also an optional configured attribute with the
configuration conditions that are the same as those of stub areas.
When a stubby area or totally stubby area is configured, the following must be
noted:
■ The backbone area cannot be configured as a stubby area or totally stub area
and the virtual connection cannot pass through a stubby area or totally stub
area.
■ If one area is configured as stubby area or totally stub area, all routers in this
area must be configured with this attribute.
■ An ASBR cannot be inside a stubby area or a totally stub area, which means
that the exterior route of the AS cannot be transferred to the area.
Perform the following configuration under OSPF view:
Table 498 Configure Totally Stubby Area of OSPF
By default, no stubby area or totally stub area is configured. The cost of the
default routing sent to Stub area is 1.
The area is configured of totally stub area when no-summary option is selected
Configuring an NSSA
Area
NSSA areas are areas that can import external routing by itself and advertise in the
AS, but cannot accept external routing generated by another area in the AS.
Actually an NSSA area is one form of a stub area, which can conditionally import
AS external routing. A new area-NSSA Area and a new LSA-NSSA LSA (or called
Type-7 LSA) are added in the RFC1587 OSPF NSSA Option.
The NSSA and stub area are similar in many ways. Neither of them generates or
redistributes an AS-External-LSA (namely Type-5 LSA), and both of them can
generate and import a Type-7 LSA. Type-7 LSA is generated by the ASBR in an
NSSA area, which can only advertise in an NSSA area. When a Type-7 LSA reaches
the ABR of an NSSA, the ABR selects whether to transform the Type-7 LSA into an
AS-External-LSA to advertise to other areas.
Operation Command
Define an area as stub area or totally stub
area and specify cost value.
stub cost cost area area-id [
no-summary ]
Cancel Stub Area undo stub cost cost area area-id [
no-summary ]