Subject: Re: pppoe problem
To: der Mouse <mouse@Rodents.Montreal.QC.CA>
From: Rick Byers <>
List: tech-net
Date: 12/16/2001 19:41:24
On Sun, 16 Dec 2001, der Mouse wrote:

> > Is there some tool other than traceroute for finding the perpetrator,
> I don't know of any tool - including traceroute - for finding the
> perpetrator, short of sniffing traffic at various points between the
> offending endpoint and the MTU-limiting gateway and seeing when the
> "unreachable - need to frag" ICMPs start vanishing.

In my experience, the problem is often cause by some admin blindly
blocking ALL ICMP packets, so finding the point at which ICMP echo
requests get discarded (i.e. using traceroute and ping, or just
traceroute in ICMP mode) is usually a pretty good guess - assuming there
is only one such router along the path anyway.  However, many networks
legitamately block ICMP echo packets without blocking the need-frag
packet, so just because ping stops working doesn't necessarily mean there
is also a PMTU blackhole there too...