Improving route diversity through the design of iBGP topologies

Cristel Pelsser, Tomonori Takeda, Eiji Oki and Kohei Shiomoto

In a Service Provider (SP) network, routes for external destinations are distributed on iBGP sessions. This traditionally required the establishment of a full-mesh of iBGP sessions in the network. A common practice is now to make use of Route Reflectors (RR). Such a practice is more scalable in the number of iBGP sessions to be configured in a SP network. However, it has been shown that RRs have a negative impact on the diversity of routes available in the network. This is an important issue as routers may not be able to quickly use an alternate route in case of a route failure.
In this paper we tackle the problem of route diversity in a Service Provider network composed of RRs. We propose an algorithm to design iBGP session topologies with improved route diversity. We rely on an initial route reflection topology. Our algorithm proposes the addition of a few iBGP sessions to some border routers of the domain. These border routers receive a large number of external routes for which routers lack diversity. We show by means of simulations that our algorithm meets its goals. In the resulting topologies, each BGP router knows at least two different ways to reach distant destinations. This is ensured as long as a prefix advertisement is received at different nodes at the border of the AS. Secondly, we observe that the number of iBGP sessions required to achieve this goal is significantly below the number of sessions required in the case of a full-mesh. Finally, the remaining lack of route diversity after the use of our design algorithm indicates that new external peering sessions should be established. In this case, our algorithm shows that diversity cannot be reached for some prefixes independently of the iBGP topology, with the current external peering sessions.

In ICC, 2008.

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