[isp-bgp] Tier-2 reachability and multihoming

[G Pavan Kumar <pavanji@...>]

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,
pavan





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