[c-nsp] Suspect MTU Issues

[Keegan Holley]

That depends on your use.  Some technologies such as certain types of
storage replication perform better without jumbo frames.  Some won't even
use them.  Generally speaking  the maximum supported by all of your devices
would be the best thing to configure as long as it doesn't break some other
application.

2011/7/13 Leigh Harrison <lharrison at convergencegroup.co.uk>

> This discussion brings me neatly onto my follow on question then:-
>
> On the ME3600X switches they will allow me to set interface mtu of up to
> 9800 bytes. Some of my team are arguing that we only need 1548, some are
> saying 1600.
>
> We've got dark fibre, so should we be going for the maximum mtu size
> possible on the box (taking into account max mtu of the box it plugs into),
> or is there a good "all rounder" for mtu size?
>
> What mtu will cause me the least pain in years to come?
>
> Thanks for all of the valuable insight so far.
>
> Leigh
>
> Sent from my iPhone - apologies for any spelling or grammar mistakes
>
> On 13 Jul 2011, at 18:57, "Keegan Holley" <keegan.holley at sungard.com>
> wrote:
>
> > 2011/7/13 Gert Doering <gert at greenie.muc.de>
> >
> >> Hi,
> >>
> >> On Wed, Jul 13, 2011 at 09:38:56AM -0400, Keegan Holley wrote:
> >>>> You have an MTU problem.  If you want to send (1500 byte + extra
> header
> >>>> bytes) packets over a link with a MTU of 1500 -> FAIL.
> >>>>
> >>>> It's actually going to be 1500 - header sizes.  So 1500 - MPLS
> (4bytes)
> >> =
> >>> 1496  possibly - IPSEC (20 Bytes) = 1476
> >>
> >> The input packet is 1500 bytes (+ethernet headers, not counted on IOS
> MTU
> >> settings), you tack 4 byte MPLS on it -> your egress packet is 1504
> >> (+ethernet header).  So if your intermediate switches only allow
> >> 1500, you have a FAIL.
> >
> >
> > I just wanted to show that 1500 bytes is too big as is 1499, 1498, and
> 1497,
> > which was also part of the original question.  Also, that it get's worse
> > with IPsec or other protocols that would add headers such as tunneled
> IPv6.
> > We are ultimately saying the same thing.  It's not good to run MPLS with
> > the MTU set to 1500.
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