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CLI-based Configuration Guide - IP Unicast Routing

AR100, AR120, AR150, AR160, AR200, AR1200, AR2200, AR3200, and AR3600 V200R009

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Configuring OSPF NSSA

Configuring OSPF NSSA

Configuring a non-backbone area on the border of an autonomous system (AS) as a not-so-stubby area (NSSA) can reduce entries in the routing table and the amount of routing information to be transmitted.

Applicable Environment

To both import external routes and prevent resource consumption caused by external routes, you can configure an NSSA.

The NSSA is a special type of OSPF area. Neither an NSSA nor a stub area transmits routes learned from other areas in the AS where it resides. A stub area does not allow AS external routes to be imported, whereas an NSSA allows AS external routes to be imported and advertised in the entire AS.

Type 7 LSAs are used to carry imported AS external routing information in the NSSA. Type 7 LSAs are generated by the ASBRs of NSSAs and flooded only in the NSSAs where ASBRs reside. The ABR in an NSSA selectively translates received Type 7 LSAs into Type 5 LSAs to advertise AS external routing information to the other areas over the OSPF network.

Pre-configuration Tasks

Before configuring an NSSA, complete the following tasks:


  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 ] | suppress-default-route } | flush-waiting-timer interval-value | no-import-route | no-summary | set-n-bit | suppress-forwarding-address | translator-always | translator-interval interval-value | zero-address-forwarding | translator-strict ]*

    The specified area is configured as an NSSA.

    • NSSA attributes must be configured on all devices in the NSSA using the nssa command.
    • Configuring or deleting NSSA attributes may update the routing information in the area and disconnect neighbor relationships. NSSA attributes can be reconfigured or deleted only after the routing update is complete.
    The nssa command is applicable to the following scenarios:
    • The default-route-advertise parameter is configured to advertise Type 7 LSAs carrying the default route on the ASBR to the NSSA.

      Regardless of whether the default route exists in the routing table on the ABR, Type 7 LSAs carrying the default route will be generated. However, Type 7 LSAs carrying the default route will be generated on the ASBR only when the default route exists in the routing table.

    • 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.

    • When the area to which the ASBR belongs is configured as an NSSA, invalid Type 5 LSAs from other routers in the area where LSAs are flooded will be reserved. These LSAs will be deleted only when the aging time reaches 3600s. The router performance is affected because the forwarding of a large number of LSAs consumes memory resources. To resolve such a problem, you can set the parameter flush-waiting-timer to the maximum value 3600s for Type 5 LSAs so that the invalid Type 5 LSAs from other routers can be deleted in time.

      • When the LS age field value (aging time) in the header of an LSA reaches 3600s, the LSA is deleted.

      • If an ASBR also functions as an ABR, flush-waiting-timer does not take effect. This prevents Type 5 LSAs in the non-NSSAs from being deleted.

    • If an ASBR also functions as an ABR, the no-import-route parameter can be 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 that are transmitted to the NSSA. This implementation 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.
    • If multiple ABRs are deployed in the NSSA, the system automatically selects an ABR (generally 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 period.

  5. (Optional) Run default-cost cost

    The cost of the default route on which Type 3 LSAs are transmitted to the NSSA by the ABR is set.

    To ensure the reachability of AS external routes, the ABR in the NSSA generates a default route and advertises this route to the other routers in the NSSA. The cost of the default route to an NSSA is set and the selection of the default route is adjusted.

    Type 7 LSAs can be used to carry default route information to guide traffic to other ASs.

    Multiple ABRs may be deployed in an NSSA. To prevent routing loops, ABRs do not calculate the default routes advertised by each other.

    By default, the cost of the default route to the NSSA by the ABR is 1.

Verifying the Configuration

Run either of the following commands to check LSDB information:

  • display ospf [ process-id ] lsdb [ brief ]

  • display ospf [ process-id ] lsdb [ { router | network | summary | asbr | ase | nssa | opaque-link | opaque-area | opaque-as } [ link-state-id ] ] [ originate-router [ advertising-router-id ] | self-originate ] [ age { min-value min-age-value | max-value max-age-value } * ]

Run either of the following commands to check routing table information:

  • display ospf [ process-id ] routing [ ip-address [ mask | mask-length ] ] [ interface interface-type interface-number ] [ nexthop nexthop-address ]

  • display ospf [ process-id ] routing router-id [ router-id ]

Run the display ospf [ process-id ] interface [ all | interface-type interface-number ] [ verbose ] command to check OSPF interface information.

Updated: 2019-12-27

Document ID: EDOC1000174069

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