Clusterlist Attribute

The CLU STgR_LIST attriente, also defined in RFC 2796, is an optional n ontransit i ve attri bhte used to prevent loops when m ore than oue eoute reflector clester exists within an AS. The CLUSTERJJST is a 4-byte value that contains a list of CLUSTERED values that describe the teflecrlon path that a route passed through, similuFto the AS_PATH attribute. Saimilar t:o tlu^ ORIGINATOR_ID, t he CLUSTER_ID is the BGP ID oo the ro uter. Whea a rou te refleuto r receives an update, it checks the; value of tue CLUSTER_LIST attribute. If the CLUSTER_LIST field is empty, it adds its CLUSTER_ID to the field. If the field contains other entries, it prepends its local CLUSTER_ID to the list. If a route reflector receives an update with its own CLUSTER_ID in the CLUSTER_LIST, it assumes a routing loop exists and ignores the update. Figure 7-39 demonstrates how the CLUSTER_ID is prepended to the CLUSTER_LIST to prevent routing loops within an AS.

Figure 7-39. CLUSTER_LIST Attribute

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