increasing the ethernet MTU greater than 1500 (1502)

[Richard A Steenbergen]

On Fri, Jan 06, 2006 at 10:02:07AM -0800, John-Mark Gurney wrote:
> Nope, for pre-gige, only 1500 MTU is supported...  This was extended
> slightly to support vlan tagging, but I believe many of the drivers to
> magic behind the scenes to know when the vlan tagged packets are comming,
> and let them through...  You could look at the drivers into fixing this..
> If you have a good gige card, you can go to 9000 MTU and beyond...
> 
> there is still the problem of the receiving station.. (and all switches,
> routers inbetween)...   They must be able to support the larger MTU
> packet, otherwise, it coulde get dropped from the wire as too long of
> a packet...

Technically speaking, the problem was not fixed by GigE or even 10GigE. 
The wonderful folks over at the IEEE feel there is no need to bring us out 
of the ethernet networking dark ages by defining any kind of standards for 
anything > 1500 bytes. We're left with a bunch of vendors who are each 
trying to do the right thing by picking random sizes ranging from 1518 
(stock + 4 bytes for a single .1q tag) to 16384, but there are no real 
standards, no mechanisms for ensuring interoperability, no protocols for 
negotiating MTUs between networks, etc.

GigE and beyond is statistically more likely to support some kind of jumbo 
frame just because it is newer, but the only way to find out is to ask the 
vendor/manufacturer. Lots of newer FastE cards support jumbos or some kind 
of mini jumbo too. There are still plenty of NICs and switches out there 
with no or very half-ass jumbo support though.

-- 
Richard A Steenbergen <ras at e-gerbil.net>       http://www.e-gerbil.net/ras
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