North American Network Operators Group
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Re: Seeking Comcast Contact: need to troubleshoot packet loss and/or asymmetric routing issue between Comcast & Onvoy

  • From: Robert Boyle
  • Date: Thu Aug 02 11:53:43 2007

At 09:30 AM 8/2/2007, Craig D. Rice wrote:
For four months dozens of our users who are Comcast subscribers have had difficulty reaching St. Olaf College's and Carleton College's network services.

We have worked through everything we can think of with our Onvoy (regional ISP) network engineers. We have isolated the problem a couple of Comcast's IP subnets, but need a contact within Comcast to further troubleshoot.

(snip)

Either your firewall/router or the customer's firewall/router is blocking PMTUD packets. Fragment needed, but don't fragment bit set. Look at your ICMP access list and make sure you are allowing: permit icmp any any unreachable from any Internet address. I suspect an overzealous firewall admin is blocking all icmp. Read the acronym to him/her and explain that some icmp is necesary for the Internet to work.

-Robert



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