Re: Can you explain why paths to same host diverge?

[Mark Boolootian <booloo () cats ucsc edu>]

John,

Thanks very much for providing the cogent explanation.

You ask:
> I feel obligated to ask -- is there some reason you didn't
> direct your query to Sprint, before asking NANOG? It really seems like
> this is the kind of question they should be able to answer for you,
> and diagnose the problem to some extent. I can't see a good reason to ask
> here without asking the providers in question, first.

There were really two reasons I asked this question here.  First, it 
seemed like an interesting operational issue that I hadn't ever seen
beaten to death on NANOG.  Everyone is used to asymmetry between forward
and reverse paths, but I don't think I'd ever seen a case of asymmetry
in the forward path (at least, not while the network was stable).  Second,
I don't necessarily expect my provider to tell me why things route the way
they do; I only expect them to fix things when they're broken.  NANOG
seems the appropriate place to ask the "why" questions.  For what it's
worth, I am pursuing this with our provider.

> It is worth noting, I suppose, that optioned packets (i.e. traceroute -g
> or ping -R) are not CEF-switched, and therefore cannot be used to
> instrument the behavior of this hash. As a result, your best bet is
> limited ttl probes to various hops.

Presumably this is why the reverse traceroutes I ran didn't seem to
shed any light.

Thanks again for the end2end-interest pointer and the fine explanation.

regards,
mb
-- 
Mark Boolootian 
booloo () cats ucsc edu