Windows 2008R2 MTU reverts to default

[Christopher Werny]

Hi,

see below.

On 10 June 2013 22:48, Phil Pennock <ipv6-ops+phil at spodhuis.org> wrote:
>> On 2013-06-10 at 14:57 +0200, Dick Visser wrote:
>>> This works for a while, but after a minute or so it changes back to the default:
>>
>>> Any ideas what might be causing this?
>>
>>A thought, so only qualifies as "might", I don't know Windows to speak
>>definitively, but ... IPv6 NDP Route Advertisement with the MTU option?
>>
>>Windows then updating the manually-configured value based upon learnt
>>values on the wire?

>Yup, this was the case. I watched it continuously, and when an RA came
>in, it overwrote the manually configured MTU.
>Next question: how do I prevent that from happening?

Given that these systems are Windows Server 2008 R2, I would assume (and I can be wrong here) that you are configuring IPv6 related settings manually (address, gateway/dns etc.).
One potential way _could_ be the deactivation of RA processing on these systems with the following command:

"netsh int ipv6 set int [index] routerdiscovery=disabled".

I don't know whether this is feasible in/for your environment, but (at least as of right now) it is the only solution which comes to my mind. I would also be interested whether this behavior you experienced is "working as intended" or could be considered a bug. Given that you mentioned that Windows 7 keeps the MTU value (haven't checked myself) I would (maybe naively)  assume it is a bug. Will test this tomorrow with Windows 7 and Server 2008 R2.

Cheers,
Chris