Configuring an OSPFv3 NSSA

By configuring a non-backbone area on the border of an autonomous system (AS) as a not-so-stubby area (NSSA), you can reduce the size of the routing table and the amount of routing information to be transmitted.

Usage Scenario

An excessive number of entries in a routing table wastes network resources and causes high CPU usage. To reduce the number of entries in a routing table and the amount of routing information to be transmitted, configure a non-backbone area on the border of an AS as a stub area or an NSSA. For details about how to configure an OSPFv3 stub area, see Configuring an OSPFv3 Stub Area.

OSPFv3 stub areas cannot import external routes from outside an AS, nor can they learn the external routes imported by the other areas in the same AS. To import external routes to an area and minimize resource consumption, configure the area as an NSSA. NSSAs can import the external routes (outside an AS) and advertise them within the entire AS, without learning external routes from the other areas in the AS. This reduces the consumption of bandwidth and storage resources on the router.

NSSA attributes must be configured on all routers in an NSSA.

Pre-configuration Tasks

Before configuring an OSPFv3 NSSA, complete the following tasks:

Configure an IP address for each interface to ensure that neighboring devices can communicate with each other at the network layer.
Configure basic OSPFv3 functions.

Procedure

Run system-view
The system view is displayed.
Run ospfv3 [ process-id ]
The OSPFv3 process view is displayed.
(Optional) Run lsa-forwarding-address { standard | zero-translate }
The OSPFv3 forwarding address (FA) function is enabled.
Run area area-id
The OSPFv3 area view is displayed.
Run nssa [ { default-route-advertise [ backbone-peer-ignore | cost cost-value | type type-value | tag tag-value ] ^* } | no-import-route | no-summary | translator-always | translator-interval interval-value | set-n-bit | suppress-forwarding-address ] ^*
The specified area is configured as an NSSA.

The usage scenarios of the nssa command parameters are as follows:
- To allow default Type 7 LSA generation, specify default-route-advertise. After this parameter is specified, a Type 7 LSA carrying the default route (::/0) will be generated on an ABR, regardless of whether the route ::/0 exists in the ABR's routing table. On an ASBR, however, such an LSA can be generated only if the route ::/0 exists in the ASBR's routing table.
- If an ASBR also functions as an ABR, specifying no-import-route prevents OSPFv3 from advertising the external routes imported using the import-route command to the NSSA.
- To reduce the number of LSAs to be transmitted to the NSSA, specify no-summary on an ABR. This prevents the ABR from transmitting Summary LSAs (Type 3 LSAs) to the NSSA.
  
  After the nssa default-route-advertise backbone-peer-ignore no-summary command is run, the ABR generates both the default Type 7 LSA and default Type 3 LSA as long as the backbone area contains interfaces that are up. The ABR generates such LSAs regardless of whether neighbor relationships in Full state exist. In this case, the default route carried in the Type 3 LSA has a higher priority.
- To set the value of the N-bit flag to 1 in DD packets sent by the router, specify the set-n-bit parameter.
- If multiple ABRs are deployed in the NSSA, OSPFv3 automatically selects an ABR (generally the ABR with the largest router ID) as a translator to translate Type 7 LSAs into Type 5 LSAs. A change of translator causes LSA flooding. To prevent this issue, run the nssa command with translator-always specified on an ABR to configure the ABR as a fixed translator. You can also use this parameter on multiple ABRs to configure them as fixed translators. In this case, Type 5 LSAs received from the fixed-translator ABRs can be used to calculate ECMP paths.
- To ensure service continuity during translator switching, specify the translator-interval parameter. The value of interval-value must be greater than the flooding interval.
- For Type 5 LSAs converted from Type 7 LSAs, to disable such LSAs from carrying forwarding address, specify the suppress-forwarding-address parameter.
Run commit
The configuration is committed.

Verifying the Configuration

After configuring an OSPFv3 NSSA, verify the configuration.

Run the display ospfv3 [ process-id ] routing [ [ ipv6-address prefix-length ] | abr-routes | asbr-routes | ase-routes | inter-routes | intra-routes | nssa-routes ] [ verbose ] command to check the OSPFv3 routing table information.
Run the display ospfv3 [ process-id ] lsdb [ area area-id ] [ [ originate-router advertising-router-id | hostname hostname ] | self-originate ] { grace | inter-prefix | inter-router | intra-prefix | link | network | router | router-information | nssa } [link-state-id ] [ age { min-value min-age-value | max-value max-age-value } ^* ] command to check the OSPFv3 LSDB information.