Cisco IOS IPv6 TCP MSS adjustment (from orignial thread: option 212 for 6RD)

Trevor Warwick (twarwick) twarwick at
Fri May 10 15:25:12 CEST 2013

I thought it would be worth giving an update on a couple matters arising from this original thread.

- The behaviour Mikael noticed, where MSS adjustment wasn't working for TCP over IPv6, was a bug that was specific to the 7300 platform. The fix is now committed and will ship in the next maintenance update of 15.2(4)M.

- Partly as a result of the feedback received on the original thread, there is a revised implementation of TCP MSS adjustment planned initially for 15.3(3)M for ISR platforms, which will allow MSS for TCP over IPv4 and IPv6  to be adjusted independently. This implementation will then appear on the ASR1000 series in a subsequent release, currently intended to be 15.4(1)S. [usual disclaimer about not taking these plans as commitments applies, if you have a need for any of these capabilities in a particular timeframe, please work with your account team to get formal agreements for delivery]

Trevor Warwick
Director, Cisco NOSTG Software Engineering, UK

-----Original Message-----
From: Trevor Warwick (twarwick) 
Sent: 25 January 2013 10:05
To: 'Mikael Abrahamsson'
Cc: Ivan Pepelnjak; ipv6-ops at
Subject: RE: option 212 for 6RD

OK, let me look at this offline and get back to you. The code is definitely present in that version, so it's either a bug or a config. issue.


-----Original Message-----
From: Mikael Abrahamsson [mailto:swmike at]
Sent: 25 January 2013 08:52
To: Trevor Warwick (twarwick)
Cc: Ivan Pepelnjak; ipv6-ops at
Subject: RE: option 212 for 6RD

On Thu, 24 Jan 2013, Trevor Warwick (twarwick) wrote:

> In IOS 15.2(4)M and later, the "ip tcp adjust-mss" command will also 
> cause the same configured value to be used for adjusting IPv6 TCP 
> connections.

I tried this on my 6rd tunnel interface and it didn't work (no change seen on IPv6 MSS in either direction).

After reading your email I also tried to configure it on the uplink interfaces (they do IPv4/IPv6 native forwarding, no MPLS), and it didn't seem to make any difference (command used was "ip tcp adjust-mss 1200").

Cisco IOS Software, 7301 Software (C7301-ADVIPSERVICESK9-M), Version 15.2(4)M2, RELEASE SOFTWARE (fc2)


Mikael Abrahamsson    email: swmike at

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