Tier-2 reachability and multihoming

Wed Mar 23 11:24:15 UTC 2005

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,