[c-nsp] MTU on GigE

[Dave Temkin]

That's the wacky thing - if you have an IP MTU higher than the physical
interface MTU, it'll fragment IP packets down to the physical MTU size...
But, if you've got MPLS packets bigger (by setting mpls mtu) than the
physical interface size, it magically works...


On Sun, 6 Nov 2005, Phillip Vandry wrote:

> On Sun, Nov 06, 2005 at 07:52:27PM -0500, Alex Rubenstein wrote:
> > Question is -- does it work?
> >
> > For instance, say you have a 7206, NPE-400, running 12.0S. It has a IO-FE.
> > So, int f0/0 can only have a 'mtu' of 1500, but you can set the
> > 'tag-switching mtu' to something much higher (say 9000).
> >
> > Suppose further, that you are doing some AToM on this 7206, and the
> > interface you are AToM'ing is a DS3, with a MTU of 4470.
> >
> > Will it work? My limited memory remembers that this did work in one
> > instance, but I can't say for sure.
>
> I have not done this, but based on what you remember and also Dave
> & Chad's experiences, it seems it can work. In a way, that is a
> shame because it implies that the interface is perfectly capable of
> passing such large packets and yet there is no way to make use of that
> capability for IP: "ip mtu" is locked to 1500 for FE according to the
> document I am reading, and even if it isn't, the physical interface
> will reject the packets because the interface MTU is unchangeable
> and is only slightly higher than 1500. This is where I read that tag
> switched packets are exempt from the interface MTU. On the other hand,
> I am skeptical of this document document because it also states that
> IP packets that are accepted by the interface ("mtu") but exceed the
> IP mtu ("ip mtu") may be fragmented on input, which I doubt.
>
> > I thought the MTU issue on the FE cards of cisco was a physical one -- if
> > so, why does this work? What is the real reason cisco fe cards can't do
> > jumbo? non-technical?
>
> There must be a physical limit somewhere:
>
> xxxx(config-if)#ip mtu ?
>   <68-1000000>  MTU (bytes)
>
> surely I cannot successfully use 1MB packets!
>
> -Phil
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