MTU handling in 6RD deployments

Anfinsen, Ragnar Ragnar.Anfinsen at
Thu Jan 9 14:35:22 CET 2014

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
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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