[CALUG] Gigabit Ethernet

Tim Thomas tithomas1 at comcast.net
Wed Oct 19 16:54:37 CDT 2005

Hi Ray.  You're right that all equipment in a local LAN (NICs,
switches) needs to support jumbo frames if you intend to use them.
The problem is that non-jumbo equipment will see a jumbo frame
as a "jabber" and will effectively lop off anything above the standard
data size of 1500 bytes.  That screws up the CRC check and what's
left of the frame would be discarded anyway.

Note jumbo frames are not part of the official 802.3z standard.  But
most network vendors support them anyway, mainly for increased
server-to-server transfer performance.  It takes less DMA/OS
overhead to deal with chunks of 9000 bytes (typical jumbo size)
vs lots more 1500 byte chunks.

On Oct 19, 2005, at 8:33 AM, Ray Lischner wrote:

> On Tuesday 18 October 2005 07:00 am, you wrote:
>> Wouldn't PMTU discovery allow the sending host to retransmit at 1500
>>   for anything not destined for the local LAN?
> Upon further reflection, I'm still faced with the problem that reports
> from the field say that jumbo doesn't work--even in a LAN (or is that
> especially?)--unless all hardware is jumbo-enabled.
> -- 
> Ray Lischner, Author of C++ in a Nutshell
> http://cpphelp.com
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