Seeking Comcast Contact: need to troubleshoot packet loss and/or asymmetric routing issue between Comcast & Onvoy

Robert Boyle robert at
Thu Aug 2 15:49:44 UTC 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.


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.


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