Configuring an NSSA

Procedure

1. Run system-view

   The system view is displayed.

2. Run ospf [ process-id ]

   The OSPF process view is displayed.

3. Run area area-id

   The OSPF area view is displayed.

4. Run nssa [ { default-route-advertise [ backbone-peer-ignore ] } | no-import-route | no-summary | set-n-bit | suppress-forwarding-address | translator-always | translator-interval interval-value | zero-address-forwarding ] ^*

   The area is configured as an NSSA.

   

   • NSSA attributes must be configured on all routers in the NSSA using the nssa command.
   • Configuring or deleting NSSA attributes may trigger routing update in the area and disconnection from neighbors. The NSSA attributes can be reconfigured or re-canceled only after the routing update is complete.

   The nssa command is applicable to the following scenarios:

   • Default Type 7 LSAs are generated and advertised to an NSSA.

     1. OSPF neighbor relationships that are in the Full state and interfaces that are Up exist in the backbone area.
     2. The default-route-advertise parameter is configured.
     3. Other default routes destined for 0.0.0.0 exist in the local routing table.

     If condition 1 is met, ABRs generate default Type 7 LSAs and advertise them to the NSSA. If conditions 2 and 3 are met, ASBRs generate default Type 7 LSAs and advertise them to the NSSA.

   • The parameter backbone-peer-ignore is used to prevent the ABR from checking the neighbor status when the ABR generates default Type 7 LSAs and advertises them to the NSSA. Specifically, the ABR generates default Type 7 LSAs and advertises them to the NSSA as long as an interface that is Up exist in the backbone area.

   • An ASBR also functions as an ABR, and the no-import-route parameter is configured to prevent external routes imported using the import-route command from being advertised to the NSSA.
   • The no-summary parameter is configured on an ABR to reduce the number of LSAs to be transmitted to the NSSA, which prevents the ABR from transmitting Type 3 LSAs to the NSSA.
     

     After the nssa default-route-advertise backbone-peer-ignore no-summary command is run, the ABR generates default Type 7 and Type 3 LSAs as long as an interface that is Up exist in the backbone area. The default Type 3 LSAs preferentially take effect.

   • After the set-n-bit parameter is configured, the N-bit is set in the database description (DD) packets during the synchronization between the router and neighboring routers.
   • The suppress-forwarding-address parameter sets the forwarding address (FA) of the Type 5 LSAs translated from Type 7 LSAs by the NSSA ABR to 0.0.0.0.
   • If multiple ABRs are deployed in the NSSA, the system automatically selects an ABR (the router with the largest router ID) as a translator to convert Type 7 LSAs into Type 5 LSAs. You can configure the translator-always parameter on an ABR to specify the ABR as an all-the-time translator. To specify two ABRs for load balancing, configure the translator-always parameter on the chosen ABRs to specify the ABRs as all-the-time translators. You can use this command to pre-configure a fixed translator to prevent LSA flooding caused by translator role changes.
   • The translator-interval parameter is used to ensure uninterrupted services when translator roles change. The value of interval-value must be greater than the flooding interval.
   • The zero-address-forwarding parameter is used to set the FA of the generated NSSA LSAs to 0.0.0.0 when external routes are imported to the ABR in an NSSA.

5. (Optional) Run default-cost cost

   The cost of the default route carried in Type 3 LSAs that are transmitted by the ABR to the NSSA is set.

   To ensure the reachability of AS external routes, the ABR in the NSSA generates a default route and advertises it to other routers in the NSSA.

6. Run commit

   The configuration is committed.