Comcast enables 6to4 relays

Franck Martin franck at
Mon Aug 30 22:33:03 UTC 2010

if only I had a static IPv4 address, but who gives such luxury for free nowadays to a end user?

I'm in an end user configuration here

The office configuration is working perfectly fine.

I'm talking about my experience as a "end user" just enabling IPv6 on my airport extreme and trying to figure out what is happening....

On my return path I got:
traceroute from 2001:470:1:74::3 to 2002:7114:4a9d:0:xxxxxxxx
    1  2001:470:1:74::1  5.582 ms [mtu: 1500]
    2  2001:470:0:2d::2  0.483 ms [mtu: 1500]
    3  2001:470:0:30::2  173.747 ms [mtu: 1500]
    4  2001:470:0:13b::2  0.848 ms [mtu: 1500]
    5  2002:7114:4a9d::1  274.299 ms [mtu: 1480]
    6  2002:7114:4a9d:0:xxxxxxxx  299.939 ms [*mtu: 1422]

So I suspect on return path I use a HE.Net relay?

And yes I agree it is such a PITA to troubleshoot, I cannot pinpoint the issue. Nor I cannot see if I'm the only one with this kind of troubles, I suspect most just give up immediately...

----- Original Message -----
From: "Jack Bates" <jbates at>
To: "Franck Martin" <franck at>
Cc: "John Jason Brzozowski" <john_brzozowski at>, "NANOG" <nanog at>
Sent: Tuesday, 31 August, 2010 10:14:39 AM
Subject: Re: Comcast enables 6to4 relays

Franck Martin wrote:
> Well I found my 6to4 gateway:
<snip trace>
> and I have so much issues with 6to4 that I have decided to disable it at home (airport extreme). I found out PTB was not transmitted and using scamper and the help of Matthew Luckie there is an odd MTU of 1422 from Internet to me. I suspect the 6to4 relay did not put the MTU to 1280 to be on the safe side... (I saw a recommendation like that on the net).

Actually, their MTU won't matter. 6to4 is EXTREMELY asymmetric when 
using relays. The return traffic to you will be via the first router 
than can support 6to4, not the relay you sent packets to. This is why 
troubleshooting 6to4 is such a PITA. If your airport supports static 
tunnels, set one up at tunnel brokers and be done with it.


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