Posts Tagged ‘anomalies’



In this series of posts, we are going to review some interesting topics illustrating unexpected behavior of the BGP routing protocol. It may seem that BGP is a robust and stable protocol, however the way it was designed inherently presents some anomalies in optimal route selection. The main reason for this is the fact that BGP is a path-vector protocol, much like a distance-vector protocol with optimal route selection based on policies, rather than simple additive metrics.

The fact that BGP is mainly used for Inter-AS routing results in different routing policies used inside every AS. When those different policies come to interact, the resulting behavior might not be the same as expected by individual policy developers. For example, prepending the AS_PATH attribute may not result in proper global path manipulation if an upstream AS performs additional prepending.

In addition to that, BGP was designed for inter-AS loop detection based on the AS_PATH attribute and therefore cannot detect intra-AS routing loops. Optimally, intra-AS routing loops could be prevented by ensuring a full mesh of BGP peering between all routers in the AS. However, implementing full-mesh is not possible for a large number of BGP routers. Known solutions to this problem – Route Reflectors and BGP Confederations – prevent all BGP speakers from having full information on all potential AS exit points due to the best-path selection process. This unavoidable loss of additional information may result in suboptimal routing or routing loops, as illustrated below.

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