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Configuration Guide - IP Unicast Routing

S9300, S9300E, and S9300X V200R011C10

This document describes IP Unicast Routing configurations supported by the switch, including the principle and configuration procedures of IP Routing Overview, Static Route, RIP, RIPng, OSPF, OSPFv3, IS-IS(IPv4), IS-IS(IPv6), BGP, Routing Policy ,and PBR, and provides configuration examples.
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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 Autonomous System Boundary Routers (ASBRs) of NSSAs and flooded only in the NSSAs where ASBRs reside. An Area Border Router (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 disconnect neighbor relationships. NSSA attributes can be reconfigured or deleted only after the routing update is complete.
    The nssa command parameters apply to the following scenarios:
    • The default-route-advertise parameter allows an ASBR to advertise a default route carried in the Type 7 LSA to the NSSA.

      Regardless of whether the routing table on the ABR contains the default route, a default route carried in the Type 7 LSA is generated. However, a default route carried in the Type 7 LSA is generated on an ASBR only when the routing table contains the default route

    • When the area to which the ASBR belongs is configured as an NSSA, invalid Type 5 LSAs from other switches in the area are deleted only when the aging time reaches 3600s. Because the forwarding of a large number of LSAs consumes memory resources, the switch performance is degraded. To resolve this problem, set the parameter flush-waiting-timer so that the aging time of the invalid Type 5 LSAs from other switches reaches 3600s and are deleted immediately.

      • 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 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 if there is an Up interface 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 switch and neighboring switches.
    • If multiple ABRs are deployed in the NSSA, the system automatically selects an ABR (generally the switch with the largest router ID) as a translator to convert Type 7 LSAs into Type 5 LSAs. To specify one or two ABRs (in load balancing scenarios) as all-the-time translators, configure the translator-always parameter on the chosen ABRs. 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 the Type 3 LSA that the ABR sends to the NSSA 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 switches 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 exist 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 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: 2019-10-22

Document ID: EDOC1000178418

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