Re: [RAM] Tunnelling & GigE jumbo frame size

Hi,

On Wed, Sep 12, 2007 at 04:07:41PM -0400, RJ Atkinson wrote:
> >>o All router-to-router links on the downstream side of an ITR use
> >>either 4470 or 9180 byte MTUs.
> >
> >Where is this assumption coming from?
> >As long as there are FastE links, this is not going to happen.
> 
> Are you trying to say that you currently have deployed
> router-to-router links inside an ISP/IX with less than
> 4470 bytes of IP MTU ?

We have no single ethernet link in our network that has a MTU *above* 1530
(there are some that could be cranked up, but there was no need yet)

I have a large number of links that have a MTU of *exactly* 1500 (because
there is equipment that cannot handle 1530).

All exchange points that we're connected to run the fabric at 1500 byte
MTU, because they have members that have equipment that cannot handle
more.  There are *some* IXPs that have two different LANs, one with
1500 and one with "Jumbo", but that's not very widespread yet.

So: the answer is "yes".

> Btw, at least some 100baseTX/FX switches/routers can
> support the 9180 IP MTU.  So it is not necessarily
> the case that 100baseTX/FX implies a 1518 byte link MTU.

Switches are easy.  Routers are not.

What good is having a Jumbo-capable switch if there is one peer on the
shared fabric that can only receive 1500-byte (+ Ethernet header) packets?

Gert Doering
        -- NetMaster
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