Comcast enables 6to4 relays

Franck Martin franck at
Mon Aug 30 22:13:10 UTC 2010

Well I found my 6to4 gateway:
traceroute to (, 30 hops max, 40 byte packets
10 ( [AS174]  244.965 ms  244.964 ms  244.952 ms
11 ( [AS174]  244.336 ms ( [AS174]  244.300 ms  266.250 ms
12 ( [AS174]  326.141 ms  326.139 ms  376.926 ms
13 ( [AS10680]  612.357 ms  612.367 ms  612.358 ms
14 ( [AS10680]  376.777 ms  319.641 ms *

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).

I have seen that the new hardware of the airport extreme has a new firmware that does more IPv6 magic, but old hardware are not yet benefiting this new firmware... Once a new firmware comes I'll re-enable and see...

PS: I found scamper to be a very good troubleshooting tool (once you know what to do) and wish it would be on all OS, like traceroute and now tracepath is...

----- 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 9:44:11 AM
Subject: Re: Comcast enables 6to4 relays

Franck Martin wrote:
> Is there a list of 6to4 relays?
> I'm curious.
> Also, I'm also curious to know if ISPs in Europe (which are more advanced in IPv6 deployment) have experienced the same issues?

Sprint has one which is absolutely horrible (or was a year or two ago). 
I'd recommend any and every ISP to setup a 6to4, even if it runs over a 
v6 tunnel to HE. Accidentally getting one from someone else can give you 
exceptionally broken 6to4 connectivity. Being anycast, I'd say 
routeviews might be a good place to check for some, but often times they 
are hidden within the networks they serve.

That being said, 6to4 itself is often horrible. It works fine if you are 
talking 6to4 direct to the remote site (vs using a relay), but relays 
often break and are hard to troubleshoot due to their nature.

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