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Tier-2 reachability and multihoming
I have been working on characterizing the internet hierarchy.
I noticed that 27% of the total possible tier-2 provider node pairs are
unreachable 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?
2 4 tier-2
/ \ / \
5 6 7 8 tier-3
here, nodes 2 and 4 have no reachability,
2 3 4
/ \ \/ \
5 6 7 8
now, node 7 is reachable from 2 and its lower level nodes, but what
node 4 and 8, and as a typical case, suppose nodes 4 and 8 have no
multihoming whatsoever, what then?