Prior to Not So Stubby Areas (NSSA),
an area needed to be defined as a stub area in order to disable it
from receiving external (Type 5) link-state advertisements (LSAs). Area Border Routers (ABRs) that connect stub areas
do not flood any external routes they receive into the stub areas.
To return packets to destinations outside of the stub area, a
default route through the ABR is used. 
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Lab
Activity |
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In this lab, you will learn how to
use OSPF with the NSSA option to redistribute RIP routes into
the OSPF core via a stub transit area. |
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RFC 1587 defines a hybrid area called the Not-So-Stubby Area (NSSA).
An OSPF NSSA is similar to an OSPF stub area but allows for the
following abilities
- Importing
(redistribution) of external routes as Type 7 LSAs into NSSAs by
NSSA Autonomous System Boundary Routers (ASBRs).
- Translation of specific
Type 7 LSAs routes into Type 5 LSAs by NSSA ABRs.
Use OSPF NSSA in the following scenarios.
- When you want to summarize
or filter Type 5 LSAs before they are forwarded into an OSPF area.
The OSPF Specification (RFC 1583) prohibits the summarizing or
filtering of Type 5 LSAs. It is an OSPF requirement that Type 5 LSAs
always be flooding throughout a routing domain. When you define an
NSSA, you can import specific external routes as Type 7 LSAs into
the NSSA. In addition, when translating Type 7 LSAs to be imported
into nonstub areas, you can summarize or filter the LSAs before
importing them as Type 5 LSAs.

- If you are an internet
service provider (ISP), or a network administrator that has to
connect a central site using OSPF to a remote site that is using a
different protocol, such as RIP or EIGRP, you can use NSSA to
simplify the administration of this kind of topology. Prior to NSSA,
the connection between the corporate site ABR and the remote router
used RIP or EIGRP. This meant maintaining two routing protocols.
Now, with NSSA, you can extend OSPF to handle the remote connection
by defining the area between the corporate router and the remote
router as an NSSA, as shown in the figure. You cannot expand the
normal OSPF area to the remote site because the Type 5 external will
overwhelm both the slow link and the remote router.
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