Re: [isp-bgp] Tier-2 reachability and multihoming

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

Tom Vest wrote:
> Hi Pavan,
>
> Can you give us a few examples of networks that you believe are 
> suffering this reachability problem?
you can find the list of unreachable pairs of ASes at
http://www.cse.iitb.ac.in/pavanji/unreachable_T2.txt

  I tried to use traceroute from one of the ASes of a pair
to a network in the other AS, and I could obtain just an
incomplete output. Although the partial output shows the
probe passage from the originating AS to a Tier-1 AS and then
to another Tier-1 AS and thats it,the traceroute couldnt get
anything else to show as if a nosy deamon out of a sea of
clinging mists has drained the probe into its smelly trash!

> Thanks,
>
> Tom
>
> Tom Vest
> Research Program Manager
> Packet Clearing House
> http://www.pch.net
>
> On Mar 21, 2005, at 12:49 AM, G Pavan Kumar wrote:
>
> > Hi there,
> >         I have been working on characterizing the internet hierarchy.z
> > 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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