[c-nsp] Suspect MTU Issues

[Keegan Holley]

2011/7/12 Gert Doering <gert at greenie.muc.de>

> Hi,
>
> On Tue, Jul 12, 2011 at 07:46:00PM +0000, Leigh Harrison wrote:
> > There is a legacy layer 2 network which has had an mpls network
> > built over it.  A link between two of the data centres is a dark
> > fibre between two Cisco 3750E switches running on the ten gig ports
> > with a variety of vlans passing over a dot1q trunk.  Both of these
> > switches are pretty much out of the box and have a standard system
> > mtu of 1500
>
> 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.
>
> gert
>
> It's actually going to be 1500 - header sizes.  So 1500 - MPLS (4bytes) =
1496  possibly - IPSEC (20 Bytes) = 1476