92 Byte ICMP Blocking Problem

Fri Sep 12 18:16:41 UTC 2003

In message <20030912175258.GB616832 at hiwaay.net>, Chris Adams writes:
>
>Once upon a time, Richard J.Sears <rsears at adnc.com> said:
>> Since then, we have been hammered with customer complaints concerning
>> the inability to talk to mail servers and ssh to their servers, as well
>> as other weird network issues, all centering around the time we started
>> blocking 92 Byte ICMP packets.
>> 
>> Has anyone else seen this, and if so, is the only resolution to stop the
>> blockage of 92 Byte ICMP Packets..?
>
>Yes.  As soon as we put the policy route map in place, we had some
>people unable to talk via SSH, SMTP, or POP3.  It was random: one person
>here in the office couldn't SSH to a particular server.  He could SSH to
>other servers, and the rest of us could SSH to the server he could not.
>We had similar experiences with SMTP and POP3.  When we took the policy
>route map back out, the problems went away.
>
>This is with IOS 12.0(25)S1 on a 7513 doing dCEF.  We put the policy
>route map on the FE interface linking this router to the POP core
>router; this router has MC-T3 interfaces and ethernets to Ascend TNTs
>and such.  The intent was to stop the 92 byte ICMP echos from reaching
>the Ascend TNTs, since several of them were rebooting constantly.

I wonder if it's a Path MTU problem.  Can you turn off Path MTU on some 
of the affected hosts and see if it solves the problem?

		--Steve Bellovin, http://www.research.att.com/~smb