Re: [vortex] Re: MTU discovery

From: Donald Becker (
Date: Tue Jun 11 2002 - 14:27:42 EST

On Mon, 10 Jun 2002, Matti Aarnio wrote:

> Date: Mon, 10 Jun 2002 11:05:13 +0300
> From: Matti Aarnio <>
> To: Robert PipCA <>
> Cc:,
> Subject: [vortex] Re: MTU discovery
> On Mon, Jun 10, 2002 at 12:45:07AM -0700, Robert PipCA wrote:
> > Hi,
> > I'm working on a project that require knowing the max MTU size
> > supported by the 3Com PCI 3c905C (Boomerang).
> > The datasheet provided by 3Com does not mention it, and I already
> > did the usual google search, but didn't find it neither.
> > Does anyone knows a "generic way" of knowing this (or chip-specific)?
> Some devices do, however, support reception (and transmit) of what
> is called "jumbograms". With boomerang you can set a register
> to contain the limit value.

A 16 bit register.. 64KB packets. There are various issues with using
large packet sizes. There is no driver that has been verified with
jumbo frames. I have been throwing driver versions at Rishi Srivatsavai
<rishis at CLEMSON.EDU> trying to sort out the issues. You might notice
the changes in 0.99W, although they don't handle the FIFO limit issues.

> Alternatively with boomerang, and
> its predecessors, you can set a bit to accept extra-large frames.
> I recall the ultimate limit is in order of 4kB.

More precisely, FDDI frame size minus the FDDI-specific bits, about

Donald Becker
Scyld Computing Corporation
410 Severn Ave. Suite 210		Second Generation Beowulf Clusters
Annapolis MD 21403			410-990-9993

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