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:

ORIGINATOR_ID

ORIGINATOR_ID is a new optional, non-transitive BGP attribute of Type code 9. This attribute is 4 bytes long and it will be created by an RR in reflecting a route. This attribute will carry the BGP Identifier of the originator of the route in the local AS. A BGP speaker SHOULD NOT create an ORIGINATOR_ID attribute if one already exists. A router that recognizes the ORIGINATOR_ID attribute SHOULD ignore a route received with its BGP Identifier as the ORIGINATOR_ID.

CLUSTER_LIST

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.