www.arin.net

[Pekka Savola]

On Mon, 28 Aug 2006, Bernhard Schmidt wrote:
> I've seen Cisco routers (and I think I heard of Juniper routers doing
> that as well) choosing the IPv6 MTU based on the IPv4 MTU (minus
> tunneling overhead) on the egress interface towards the tunnel
> destination. Which means if your path to the tunnel destination is
> through a POS interface with an MTU of 4470 Bytes, the automatically set
> IPv6 MTU of an IPv6-in-IP tunnel is 4450 Bytes. If the complete path to
> the destination does not support 4470 Bytes, things will break.
>
> This happens quite often if a 1500 byte packet comes along and is
> encapsulated in 1520 bytes IPv4, which is then dropped somewhere along
> the path to the tunnel destination.

Correct for Junipers.  It was very advisable to manually configure the 
1480 MTU on IPv6 interfaces :-) because the tunnel implementation 
couldn't interpret and translate the received ICMPv4 PMTUD messages. 
We filed a bug on this some time ago: PR/65486 - fix is supposedly in 
7.3R3; 7.4R2; 7.5R1; 7.6R1.

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings