[outages] Reading Traceroutes (was Re: Packet loss to 4.2.2.2)

[Jay Ashworth]

----- Original Message -----
> From: "Bill Wichers" <billw at waveform.net>

> I show pretty clean to the server itself from a Level(3) connection in
> Detroit, but I was seeing loss to the router one hop up from it (which
> seems to be getting better with time).

Just a reminder to all:

intermediate hops on a traceroute may show high packet drops *without that
router actually dropping any real traffic packets*.  Many routers are 
required by their design to force ICMP traffic up into the CPU, where actual
packets may remain down on the much more efficient cross-linecard-path, which
means that if the router gets too busy doing "Real work", the ICMP will be 
the first thing to go.

Evidence of this is usually a large degree of packet loss which starts and
ends with one hop in your trace; if *everything past a certain point* seems
to be dropping roughly the same percentage of traffic, *that* is probably
the outbound link from the router.

Cheers,
-- jra
-- 
Jay R. Ashworth                  Baylink                       jra at baylink.com
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