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Configuration Guide - IP Unicast Routing

S7700 and S9700 V200R013C00

This document describes the configurations of IP Unicast Routing, including IP Routing, Static Route, RIP, RIPng, OSPF, OSPFv3, IPv4 IS-IS, IPv6 IS-IS, BGP, Routing Policy, IP Routing Table Management, and PBR.

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Configuring an OSPF NSSA

Configuring an OSPF NSSA

Applicable Environment

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

An NSSA is a special type of OSPF area. Similar to a stub area, an NSSA does not transmit external routes from other areas. Different from a stub area, an NSSA imports AS external routes and transmits them in the entire AS using Type 7 LSAs (a stub area does not import AS external routes).

Type 7 LSAs are generated by ASBRs of NSSAs and flooded only in the NSSAs where ASBRs reside. An ABR in an NSSA selectively translates received Type 7 LSAs into Type 5 LSAs to advertise AS external routes 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 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 will update the routing information in the area and interrupt neighbor relationships. NSSA attributes can be reconfigured or deleted only after the routing update is complete.
    The parameters in the nssa command apply to the following scenarios:
    • The default-route-advertise parameter is used to configure an ASBR to advertise a Type 7 LSA carrying a default route to the NSSA.

      On an ABR, a Type 7 LSA carrying a default route is generated regardless of whether the routing table on the ABR contains a default route. On an ASBR, a Type 7 LSA carrying a default route is generated only when the routing table contains a default route.

    • When the area to which an ASBR belongs is configured as an NSSA, invalid Type 5 LSAs from other switches in the area can be deleted only when the aging time reaches 3600s. These invalid LSAs consume numerous memory resources, degrading the switch performance. To resolve this problem, you can set the flush-waiting-timer parameter to generate Type 5 LSAs with the aging time set to the maximum value (3600s). The generated Type 5 LSAs with the largest aging time can delete the invalid Type 5 LSAs from other switches in time.

      • When the value of the LS age field (aging time) in the header of an LSA generated using the flush-waiting-timer parameter reaches 3600s, the LSA is deleted.

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

    • If an ASBR also functions as an ABR, you can configure the no-import-route parameter to prevent external routes imported using the import-route command from being advertised to the NSSA.
    • The no-summary parameter can be configured on an ABR to prevent an ABR from transmitting Type 3 LSAs to an NSSA so that the number of LSAs transmitted to the NSSA can be reduced.
      After the nssa default-route-advertise backbone-peer-ignore no-summary command is run, the ABR generates Type 7 and Type 3 LSAs both carrying default routes if there is an Up interface in the backbone area. The Type 3 LSAs preferentially take effect.
    • If the set-n-bit parameter is configured, the N-bit is set in the DD packets during the synchronization between the switch and neighboring switches.
    • If multiple ABRs are deployed in an NSSA, the system automatically selects an ABR (generally the switch with the largest router ID) as a translator converting Type 7 LSAs into Type 5 LSAs. To specify one or two ABRs (in load balancing scenarios) as all-the-time translators, you can configure the translator-always parameter on the selected ABRs. 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 carried in Type 3 LSAs that the ABR sends to the NSSA is set.

    To ensure reachability of AS external routes, the ABR in the NSSA generates a default route and advertises the route to other switches in the NSSA. You can use this command to set the cost of the default route to an NSSA and adjust the selection of the default route.

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

    Multiple ABRs may exist in an NSSA. To prevent routing loops, an ABR does not calculate the default routes advertised by the other ABRs.

    By default, the cost of the default route that an ABR sends to the NSSA 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 OSPF 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: 2019-04-08

Document ID: EDOC1100065744

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