Multicast and Routing Guide K/KA/KB.15.15

type-1 and type-2 LSAs and routed traffic internal to the area. ASBRs are allowed in backbone
areas.
Normal area
This area connects to the AS backbone area through one or more ABRs (physically or through a
virtual link) and supports type-3 summary LSAs and type-5 external link LSAs to and from the
backbone area. ASBRs are allowed in normal areas.
Not-so-stubby-area (NSSA)
Beginning with software release K.12.xx, this area is available and connects to the backbone area
through one or more ABRs. NSSAs are intended for use where an ASBR exists in an area where
you want to control the following:
• Advertising the ASBR's external route paths to the backbone area
• Advertising the NSSA's summary routes to the backbone area
• Allowing LSAs from the backbone area to advertise in the NSSA:
Summary routes (type-3 LSAs) from other areas•
• External routes (type-5 LSAs) from other areas as a default external route (type-7 LSAs)
In the above operation, the ASBR in the NSSA injects external routes as type 7 LSAs. (Type 5 LSAs
are not allowed in an NSSA.) The ABR connecting the NSSA to the backbone converts the type 7
LSAs to type 5 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 type-3 summary
LSAs:
• From the NSSA into the backbone area
• From the backbone into the NSSA
If the ABR detects type-5 external LSAs on the backbone, it injects a corresponding type-7 LSA
default route (0.0.0.0/0) into the NSSA
You can also configure the NSSA ABR to do the following:
• 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 150).
• Replace all type-3 summary routes and the type-7 default route with the type-3 default summary
route (0.0.0.0/0.)
Virtual links are not allowed for NSSAs.
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 external (type 5) LSAs. A stub area supports these actions:
• Advertise the area's summary routes to the backbone area.
• Advertise summary routes from other areas.
• Use the default summary (type-3) route to advertise both of the following:
Summary routes to other areas in the AS•
• External routes to other ASs
194 Open Shortest Path First Protocol (OSPF)