[Bioclusters] Jumbo frames with 100 Mb?

[Donald Becker]

On Thu, 2 May 2002, Vsevolod Ilyushchenko wrote:

> Am I correct that enabling jumbo frames on a Gb card makes sense only if
> there is another Gb card on the other end? That is, if the master has the
> GigE card, but all the slaves have only 100 Mb cards, jumbo frames will not
> help or even screw everything up, right?

This was just covered on the Beowulf list last week.
Everyone with real-life experience using jumbo frames recommends "don't
try this at home".

See the Beowulf list archives and subscription info at
   http://www.scyld.com/mailman/listinfo/

The primary consideration is the switch and jumbo frame support on the
100Mb NICs.  If the switch doesn't support jumbo frames (very few do)
you cannot use jumbo frames at all.

If the switch supports jumbo frames, but routes+fragments them to
the fast Ethernet side, you will end up with much higher latency and
lower performance.

If the switch will forward jumbo frames from Gb to 100Mb, you must have
jumbo frame support on the 100Mb hardware and driver.  There are
configurations that will support this, but you will be doing the
configuration and verification by hand.

> Also, do I have to do anyting to the switch to enable jumbo frame support?

Most definitely yes.  Jumbo frame support is incompatible with normal
configurations -- every attached device must support jumbo frames and be
explicitly configured for the size of frames you will be using.

-- 
Donald Becker				becker@scyld.com
Scyld Computing Corporation		http://www.scyld.com
410 Severn Ave. Suite 210		Second Generation Beowulf Clusters
Annapolis MD 21403			410-990-9993