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1675 Views 4 Replies Latest reply: Mar 28, 2012 7:38 PM by Bradford Chatterjee (CCIEx2/CCDE) RSS

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Mar 28, 2012 7:48 AM

rrvillarreal 28 posts since
Mar 26, 2010

Hello, i just wanted to know if Cluster-ID is really obsolete or if there are any scenarios where we want to configure the same cluster-id on different routers to avoid routing loops??

  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    1. Mar 28, 2012 9:04 AM (in response to rrvillarreal)
    Re: Cluster-ID



    Most cost companies care about uptime.    To get more uptime, they implement fault tolerance and redundancy.   If they implement redundancy regarding route reflectors, they will also need to use the cluster-id, so that a route reflector in the same cluster, can identify an update from another router reflector in the same cluster.



  • Keith Barker - CCIE RS/Security, CISSP 5,351 posts since
    Jul 3, 2009
    Currently Being Moderated
    3. Mar 28, 2012 7:27 PM (in response to rrvillarreal)
    Re: Cluster-ID

    If you are taking the certification, stick with the cluster-id. 


    RFC 4456:


    8.  Avoiding Routing Information Loops



       When a route is reflected, it is possible through misconfiguration to

       form route re-distribution loops.  The route reflection method

       defines the following attributes to detect and avoid routing

       information loops:




       CLUSTER_LIST is a new, optional, non-transitive BGP attribute of Type
       code 10.  It is a sequence of CLUSTER_ID values representing the
       reflection path that the route has passed.

       When an RR reflects a route, it MUST prepend the local CLUSTER_ID to
       the CLUSTER_LIST.  If the CLUSTER_LIST is empty, it MUST create a new
       one.  Using this attribute an RR can identify if the routing
       information has looped back to the same cluster due to
       misconfiguration.  If the local CLUSTER_ID is found in the
       CLUSTER_LIST, the advertisement received SHOULD be ignored.

    9.  Impact on Route Selection

       The BGP Decision Process Tie Breaking rules (Sect., [1]) are
       modified as follows:

          If a route carries the ORIGINATOR_ID attribute, then in Step f)
          the ORIGINATOR_ID SHOULD be treated as the BGP Identifier of the
          BGP speaker that has advertised the route.

          In addition, the following rule SHOULD be inserted between Steps
          f) and g): a BGP Speaker SHOULD prefer a route with the shorter
          CLUSTER_LIST length.  The CLUSTER_LIST length is zero if a route
          does not carry the CLUSTER_LIST attribute.



    On the service providers networks that I have worked with, they use the cluster ID in their redundant clusters. 


    It is true that the cluster related items, in the route selection process, are somewhere in the bottom 3 of a 13 step selection process.


    Any input from others is very welcome.


    Best wishes on your CCIP,



  • Bradford Chatterjee (CCIEx2/CCDE) 347 posts since
    Jul 7, 2009
    Currently Being Moderated
    Re: Cluster-ID

    If I were going to implement route reflectors in clusters, I'd use cluster IDs to be safe. It takes minutes of extra planning. I have recently spoken with engineers at other organizations running large iBGP networks specifically about route reflection, and none of them were talking about removing their cluster ids. As Keith points out, it's pretty far down the selection process, and most networks are designed such that BGP never gets to make a decision with those criteria.


    I'll also point out that RFC4456 only says "should" and not "must." It's better to be safe than sorry.


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