Redundant connectivity (or multiconnectivity) between adjacent autonomous systems (ASes) is important for interdomain traffic engineering and fast recovery in case of failures. However, the redundancy of ASes business relationship links has not been quantitatively studied, mainly due to the difficulty of obtaining relevant data. In this paper, we show that the mrinfo multicast monitoring tool can provide useful data about the Internet topology and such redundant links in particular. Our analysis relies on more than four years of daily queries to about ten thousand routers mapped into more than two hundred ASes. We demonstrate that peering links between ASes are frequently redundant. In particular, our analysis shows that more than half of the studied ASes pairs are connected through multiple physical links. We then refine our analysis by considering the different types of ASes and their business relationships. A particular result of our analysis is that at least 75% of the peer-to-peer relationships between adjacent Tier-1 ASes are redundant, i.e., the connections between these ASes involve several physical links. Our analysis is conservative, providing so a lower bound, as some links might not be seen by mrinfo due to ISPs filtering policies.

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Abstract. Due to the way BGP paths are distributed over iBGP sessions inside an Autonomous System (AS), a BGP withdraw that follows a failure may be propagated outside the AS although other routers of the AS know a valid alternate path. This causes transient losses of connectivity and contributes to the propagation of a large number of unnecessary BGP messages. In this paper, we show, based on RouteViews data, that a significant number of BGP withdraws are propagated even though alternate paths exists in another border router of the same AS. We propose an incrementally deployable solution based on BGP communities that allows the BGP routers of an AS to suspend the propagation of BGP withdraws when an alternate path is available at the borders of their AS.

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