MTU handling in 6RD deployments

Templin, Fred L Fred.L.Templin at
Thu Jan 9 16:56:58 CET 2014

Hi Ragnar,

What is the MTU as seen by the IPv6 hosts - 1480? Something less?
Would it not be better if they could see 1500+?

Thanks - Fred
fred.l.templin at

> -----Original Message-----
> From: at [mailto:ipv6-ops-
> at] On Behalf Of Anfinsen, Ragnar
> Sent: Thursday, January 09, 2014 5:35 AM
> To: IPv6 Ops list
> Subject: RE: MTU handling in 6RD deployments
> Hi all.
> We have now changed and upgraded our BR to handle MTU in a sensible manner.
> We are not able to set the IPv4 MTU to more than 1500 due to limitations in our Cisco Cat4500
> platform. So we have to reduce the MTU to 1480 somehow.
> Our other problem is that, by mistake, our RA MTU size is set to 1500, and not 1480, as it should be.
> This will be fixed in the next firmware we push out.
> Due to the problem with the MTU size, we had a lot of ICMPv6 PTB packets being returned from the Cisco
> 1K unit (BR). This again results to that the rate-limiting feature in the 1K dropped a lot of the PTB
> messages going back to the sender, hence making the IPv6 experience sluggish. Typical problems was
> that pictures and such had some timeout before they were loaded.
> To fix this issue, we first of all upgraded our 1K's to the latest IOS (Cisco IOS XE Software, Version
> 03.11.00.S) which has the MSS-Clamping feature. This feature was added by Cisco in November. Then we
> added the command "ipv6 tcp adjust-mss 1420" to the 6rd tunnel interface.
> With this command set, all ICMPv6 PTB packets has disappeared, and the whole IPv6 experience has
> become a whole lot better with snappy loading of pages and pictures. So for TCP everything is good. As
> soon as we have fixed our firmware, this problem should be gone all together.
> Thanks to Tore Anderson for point this out to us.
> Hopefully this is useful for someone.
> Best Regards
> Ragnar Anfinsen
> Chief Architect CPE
> IPv6/IPv4 Architect
> Infrastructure
> Technology
> Altibox AS
> Phone: +47 51 90 80 00
> Phone direct: +47 51 90 82 35
> Mobile +47 93 48 82 35
> E-mail: ragnar.anfinsen at
> Skype: ragnar_anfinsen
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