Windows 2008R2 MTU reverts to default

[Enno Rey]

Hi,


On Mon, Jun 10, 2013 at 10:59:21PM +0200, Dick Visser wrote:
> 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?

two things come to mind:

a) as already stated by Christopher: by deactivating the processing of RAs on the Windows systems in question.
It should be noted that this is a severe "deviation from default" (see https://www.troopers.de/wp-content/uploads/2013/01/TROOPERS13-Design+Configuration_of_IPv6_Segments_with_High_Security_Requirements-Enno_Rey.pdf for the term) and hence in general we don't like or recommend that approach.

b) (much better): have the router advertise the lower MTU you want to use in the RAs by just setting a lower MTU on the (router) interfaces in question.
See also: http://blog.ioshints.info/2013/01/mtu-issues-and-tcp-mss-clamping-in.html

best

Enno







> 
> 
> -- 
> Dick Visser
> System & Networking Engineer
> TERENA Secretariat
> Singel 468 D, 1017 AW Amsterdam
> The Netherlands

-- 
Enno Rey

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