Hi there, I have been working on characterizing the internet hierarchy. I noticed that 27% of the total possible tier-2 provider node pairs are not connected i.e., they dont have any tier-1 node connecting them nor a direct peering link between them. Multihoming can be used as a predominant reason for the reachability of tier-3 nodes which are customers of these nodes, but what about the reachability of tier-2 nodes themselves and its customers which cannot afford to multihoming? How does BGP solve this reachability problem when it gets a request to a prefix unreachable? 1 tier-1 / 2 4 tier-2 / \ / \ 5 6 7 8 tier-3 here, nodes 2 and 4 have no reachability, 1 / | 2 3 4 / \ \/ \ 5 6 7 8 now, node 7 is reachable from 2 and its lower level nodes, but what about node 4 and 8, and as a typical case, suppose nodes 4 and 8 have no multihoming whatsoever, what then? Regards,