IPv6 Configuration Guide K/KA/KB.15.15

to the AS backbone area through one or more ABRs (physically or through a virtual link). ASBRs
are allowed in normal areas.
Backbone area
Every AS must have one (and only one) backbone area (identified as area 0 or 0.0.0.0). The ABRs
of all other areas in the same AS connect to the backbone area, either physically through an ABR
or through a configured, virtual link. The backbone is a special type of normal area and serves as
a transit area for carrying the inter-area-prefix-LSAs, AS-external-LSAs, and routed traffic between
non-backbone areas, as well as the router-LSAs and network-LSAs and routed traffic internal to the
area. ASBRs are allowed in backbone areas.
Stub area
This area connects to the AS backbone through one or more ABRs. It does not allow an internal
ASBR and does not allow AS-external-LSAs. A stub area supports these actions:
• Advertise the area's inter-area routes to the backbone area.
• Advertise inter-area routes from other areas.
• Use the inter-area-prefix-LSA default route to advertise routes to an ASBR and to other areas.
You can configure the stub area ABR to do the following:
• Suppress advertising some or all of the area's summarized internal routes into the backbone
area.
• Suppress LSA traffic from other areas in the AS by replacing inter-area-prefix-LSAs and the
default external route from the backbone area with the default route (::/0).
Virtual links are not allowed for stub areas.
Not-so-stubby-area (NSSA)
This area type connects to the backbone area through one or more ABRs. NSSAs are used where
an ASBR exists in an area where you want to:
• Block injection of external routes from other areas of the AS.
• Advertise type-7-LSA external routes (learned from the ASBR) to the backbone area as
AS-external-LSAs.
NSSAs also support the following:
• Advertise inter-area-prefix-LSAs from the backbone area into the NSSA. (If no-summary is
enabled, the NSSA ABR suppresses these LSAs from the backbone and, instead, injects the
inter-area-prefix-LSA default route into the NSSA.)
• Advertise NSSA inter-area-prefix-LSAs to the backbone area.
In the above operation, the ASBR in the NSSA injects external routes as type-7-LSAs.
(AS-external-LSAs are not allowed in an NSSA.) The ABR connecting the NSSA to the backbone
converts the type-7-LSAs to AS-external-LSAs and injects them into the backbone area for propagation
to networks in the backbone and to any normal areas configured in the AS. The ABR also injects
inter-area-prefix-LSAs from the backbone area into the NSSA.
The default route (::/0) is always injected into the NSSA as either a type-7-LSA or an inter-area-LSA,
depending on the no-summary configuration (default: disabled). That is, if inter-area-prefix-LSAs
are allowed in the NSSA (the default operation), a type-7-LSA default route (::/0) is injected into
the NSSA. But if inter-area-prefix-LSAs are blocked (by enabling no-summary), the
inter-area-prefix-LSA default route is injected into the NSSA instead of the type-7-LSA default route.
You can also configure the NSSA ABR to suppress advertising some or all of the area's summarized
internal or external routes into the backbone area. See “Configuring ranges on an ABR to reduce
advertising to the backbone” (page 218).
OSPFv3 area types 253