Modeling BGP Route Selection within an AS

This paper presents a provably correct model that computes the outcome of the BGP decision process for each router in a single AS, without simulating the complex details of BGP message passing. The model requires only static inputs that can be easily obtained from the routers: the set of candidate routes to a destination (and the routers in the AS at which they were learned), import policies and other session-level parameters, and internal topology. We propose algorithms for computing route selection under four different network configurations: with the MED attribute compared across all routes, and compared only across routes from the same neighboring AS; and with a “full mesh” internal BGP (iBGP) topology versus an iBGP topology that uses a scalability technique called “route reflection”. For each scenario, we derive general properties of the routes that routers ultimately select, present an efficient algorithm for computing the outcome of BGP route selection, and prove the algorithm’s correctness. Studying the general properties and computational overhead of modeling the route selection process in each of these cases provides insights into the unnecessary complexity introduced by the MED attribute and route reflection; we use these insights to propose improvements to BGP that achieve the same goals as MED and route reflection without introducing the negative side effects of these features. We have implemented some of the algorithms from this paper in a prototype and have shown them to be efficient and accurate enough for many traffic engineering tasks.