[isp-bgp] Re: BGP path inconsistent with traceroute

[Pete Templin <petelists@...>]

Thatte, Sachin wrote:
> Can somebody shed light on the following issue?
>
> > From route-views I see as-path best path for network
> > 198.207.185.0/24 from as 3549 but the traceroute uses 701. Could it
> > be because route-views uses ebgp-multihop and the actual IP
> > next-hop router has bgp table of his own which is different than
> > that of route-views?
Each BGP router has its own table.  This means several things:

1) If at least one endpoint of your traceroute isn't located at 
route-views, you might travel over a route completely different than 
what route-views shows.
2) Routers perform destination-based forwarding.  At each hop along the 
way, the responding router forwards its response out its best path back 
to the source, so you're seeing the IP address of the interface that the 
packet is coming back from, not necessarily the IP address the packet 
was forwarded to (but you are seeing the router the packet was forwarded 
to).
3) Likewise, each of those routers along the way has a unique view back 
to the source of the traceroute.  A traceroute that started from within 
198.207.185.0/24 and headed to the point where you've been conducting 
your traceroutes might follow a completely different path than what 
you're seeing.

pt






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