IP Routing: ISIS Configuration Guide, Cisco IOS XE Release 3S
ISIS Local Microloop Protection
Downloads: This chapterpdf (PDF - 1.28MB) The complete bookPDF (PDF - 3.97MB) | The complete bookePub (ePub - 650.0KB) | Feedback

ISIS Local Microloop Protection

ISIS Local Microloop Protection

The ISIS Local Microloop Protection feature enables link-state routing protocols, such as the Intermediate System-to-Intermediate System (ISIS) protocol, to prevent or avoid local microloops during network convergence after a link-down event.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Information About ISIS Local Microloop Protection

Microloops

When changes occur in a network topology because of the failure or restoration of a link or a network device, IP Fast Reroute enables rapid network convergence by moving traffic to precomputed backup paths until regular convergence mechanisms move traffic to a newly computed best path, also known as a post-convergence path. This network convergence may cause short microloops between two directly or indirectly connected devices in the topology. Microloops are caused when different nodes in the network calculate alternate paths at different times and independently of each other. For instance, if a node converges and sends traffic to a neighbor node, which has not converged yet, traffic may loop between the two nodes.

Microloops may or may not result in traffic loss. If the duration of a microloop is short, that is the network converges quickly, packets may loop for a short duration before their TTL expires. Eventually, the packets will get forwarded to the destination. If the duration of the microloop is long, that is one of the routers in the network is slow to converge, packets may expire their TTL or the packet rate may exceed the bandwidth, and packets may get dropped.

Microloops that are formed between a failed device and its neighbors are called local uloops, whereas microloops that are formed between devices that are multiple hops away are called remote uloops. The ISIS Local Microloop Protection feature helps networks avoid local uloops. Local uloops are usually seen when there is no local loop-free alternate (LFA) path available, especially in ring or square topologies. In such topologies, remote LFAs provide backup paths for the network. However, the fast-convergence benefit of the remote LFA is at risk because of the high probability of uloop creation. The ISIS Local Microloop Protection feature can be used to avoid microloops or local uloops in such topologies.

When to Use Microloop Avoidance

The ISIS Local Microloop Protection feature supports the following local link down events

  • Interface-down events
  • Adjacency-down events due to BFD sessions going down.
  • Adjacency-down events due to neighbor holdtime expiration

The ISIS Local Microloop Protection feature can be used whether or not a topology is supported by loop-free alternates (LFAs). When you use this feature for prefixes that have repair paths installed in the forwarding plane, this feature will support interface-down events and adjacency-down events if bidirectional forwarding detection (BFD) sessions are down. If this feature is used whether or not a repair path has been installed in the forwarding plane, this feature will also support adjacency-down events caused by neighbor holdtime expiration.

The value of using this feature also depends on whether the remote event that caused loss of adjacency on the neighbor is detectable by the local forwarding plane; that is whether the forwarding plane will react and switch to using preprogrammed repair paths. For instance, when a link fails, the reaction time of the local forwarding plane depends on the media. If the media is optical, the failure is likely to be detected within milliseconds, in which case microloop avoidance is useful. If the media is copper, the local detection will be much slower or nonexistent, in which case using microloop avoidance is disadvantageous. However, if the timeout of the neighbor adjacency is due to reasons other than link failure, such as local congestion, lack of CPU time, and long input queues, these reasons are undetectable by the local forwarding plane and therefore, are not good candidates for microloop avoidance.

Note


When remote loop-free alternates (RLFAs) are enabled in a network, microloop avoidance is enabled by default for all protected prefixes (prefixes that have repair paths).

How to Configure ISIS Local Microloop Protection

Configuring Microloop Protection

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    router isis[area-tag]

    4.    microloop avoidance [ disable | protected]

    5.    end

    6.    show running-config


DETAILED STEPS
     Command or ActionPurpose
    Step 1enable


    Example:
    Device> enable
     

    Enables privileged EXEC mode.

     
    Step 2configure terminal


    Example:
    Device# configure terminal
     

    Enters global configuration mode.

     
    Step 3router isis[area-tag]


    Example:
    Device(config)# router isis
     

    Enables Intermediate System-to-Intermediate System (IS-IS) as the IP routing protocol and enters router configuration mode.

     
    Step 4microloop avoidance [ disable | protected]


    Example:
    Device(config-router)# microloop avoidance protected
     

    Enables local microloop avoidance for protected prefixes.

    Note    If you use the microloop avoidance command without any of the keywords, microloop avoidance is configured for all prefixes in the network, whether or not they are protected. The protected keyword ensures that microloop avoidance is enabled only for protected prefixes.
     
    Step 5end


    Example:
    Device(config-router)# end
     

    Returns to privileged EXEC mode.

     
    Step 6show running-config


    Example:
    Device# show running-config
     

    Displays the current running configuration.

     

    Modifying the RIB-update value

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    router isis [area-tag]

      4.    microloop avoidance[rib-update-delay delay-time]

      5.    end

      6.    show running-config


    DETAILED STEPS
       Command or ActionPurpose
      Step 1enable


      Example:
      Device> enable
       

      Enables privileged EXEC mode.

       
      Step 2configure terminal


      Example:
      Device# configure terminal
       

      Enters global configuration mode.

       
      Step 3router isis [area-tag]


      Example:
      Device(config)# router isis
       

      Enables Intermediate System-to-Intermediate System (IS-IS) as the IP routing protocol and enters router configuration mode.

       
      Step 4microloop avoidance[rib-update-delay delay-time]


      Example:
      Device(config-router)# microloop avoidance rib-update-delay 6000
       

      Configures Routing Information Base (RIB) update delay value to avoid microloops in a network.

       
      Step 5end


      Example:
      Device(config-router)# end
       

      Returns to privileged EXEC mode.

       
      Step 6show running-config


      Example:
      Device# show running-config
       

      Displays the current running configuration.

       

      Configuration Examples for ISIS Local Microloop Protection

      Example: Configuring Microloop Protection

      The following example shows how to configure microloop protection for protected prefixes:

      Device> enable
      Device# configure terminal
      Device(config)# router isis
      Device(config-router)# microloop avoidance protected
      Device(config-router)# end

      The following example shows how to configure microloop avoidance for protected and unprotected prefixes:

      Device> enable
      Device# configure terminal
      Device(config)# router isis
      Device(config-router)# microloop avoidance
      Device(config-router)# end

      The following example shows how to modify the rib-update delay:

      Device> enable
      Device# configure terminal
      Device(config)# router isis
      Device(config-router)# microloop avoidance rib-update-delay 6000
      Device(config-router)# end

      Additional References for IS-IS Local Microloop Protection

      Related Documents

      Related Topic

      Document Title

      IS-IS commands: complete command syntax, command mode, defaults, command history, usage guidelines, and examples

      Cisco IOS IP Routing: ISIS Command Reference

      Overview of IS-IS concepts

      “Integrated IS-IS Routing Protocol Overview” module

      Technical Assistance

      Description

      Link

      The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

      http:/​/​www.cisco.com/​cisco/​web/​support/​index.html

      Feature Information for ISIS Local Microloop Protection

      The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

      Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

      Table 1 Feature Information for ISIS Local Microloop Protection

      Feature Name

      Releases

      Feature Information

      ISIS Local Microloop Protection

      The ISIS Local Microloop Protection feature enables link-state routing protocols such as ISIS to prevent or avoid microloops or uloops during network convergence after a link-down event.

      The following commands were introduced or modified: microloop avoidance and microloop avoidance rib-update-delay.