Quantifying the BGP Routes Diversity Inside a Tier-1 Network

Many large ISP networks today rely on route-reflection [1] to allow their iBGP to scale. Route-reflection was officially introduced to limit the number of iBGP sessions, compared to the n×(n−1) 2 sessions required by an iBGP fullmesh. Besides its impact on the number of iBGP sessions, route-reflection has consequences on the diversity of the routes known to the routers inside an AS. In this paper, we quantify the diversity of the BGP routes inside a tier-1 network. Our analysis shows that the use of route-reflection leads to a very poor route diversity compared to an iBGP full-mesh. Most routers inside a tier-1 network know only a single external route in eBGP origin. We identify two causes for this lack of diversity. First, some routes are never selected as best by any router inside the network, but are known only to some border routers. Second, among the routes that are selected as best by at least one other router, a few are selected as best by a majority of the routers, preventing the propagation of many routes inside the AS. We show that the main reason for this diversity loss is how BGP chooses the best routes among those available inside the AS.