Chapter 19
| Unicast Routing
Configuring the Open Shortest Path First Protocol (Version 2)
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Command Usage
◆ Before creating an NSSA, first specify the address range for the area (see
“Defining Network Areas Based on Addresses” on page 717). Then create an
NSSA as described under “Adding an NSSA or Stub” on page 725.
◆ NSSAs cannot be used as a transit area, and should therefore be placed at the
edge of the routing domain.
◆ An NSSA can have multiple ABRs or exit points. However, all of the exit points
and local routers must contain the same external routing data so that the exit
point does not need to be determined for each external destination.
◆ There are no external routes in an OSPF stub area, so routes cannot be
redistributed from another protocol into a stub area. On the other hand, an
NSSA allows external routes from another protocol to be redistributed into its
own area, and then leaked to adjacent areas.
◆ Routes that can be advertised with NSSA external LSAs include network
destinations outside the AS learned through OSPF, the default route, static
routes, routes derived from other routing protocols such as RIP, or directly
connected networks that are not running OSPF.
◆ An NSSA can be used to simplify administration when connecting a central site
using OSPF to a remote site that is using a different routing protocol. OSPF can
be easily extended to cover the remote connection by defining the area
between the central router and the remote router as an NSSA.
Parameters
These parameters are displayed:
◆ Process ID – Process ID as configured in the Network Area configuration screen
(see page 717).
◆ Area ID – Identifier for a not-so-stubby area (NSSA).
◆ Translator Role – Indicates NSSA-ABR translator role for converting Type 7
external LSAs into Type 5 external LSAs. These roles include:
■
Never – A router that never translates NSSA LSAs to Type-5 external LSAs.
■
Always – A router that always translates NSSA LSA to Type-5 external LSA.
■
Candidate – A router translates NSSA LSAs to Type-5 external LSAs if
elected.
◆ Redistribute – Disable this option when the router is an NSSA Area Border
Router (ABR) and routes only need to be imported into normal areas (see
“Redistributing External Routes” on page 734), but not into the NSSA. In other
words, redistribution should be disabled to prevent the NSSA ABR from
advertising external routing information (learned through routers in other
areas) into the NSSA. (Default: Enabled)
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