Quantifying the BGP routes diversity inside a tier-1 network
Steve Uhlig and Sébastien Tandel
Many large ISP networks today rely on route-reflection 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 full-mesh.
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.
In Proceedings of Networking 2006, 2006.
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