[c-nsp] Suspect MTU Issues

[Keegan Holley]

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.