[CALUG] Gigabit Ethernet

Dave Dodge dododge at dododge.net
Tue Oct 18 16:04:10 CDT 2005

On Tue, Oct 18, 2005 at 07:00:14AM -0400, Jason Dixon wrote:
> On Oct 17, 2005, at 9:35 PM, Ray Lischner wrote:
> > For any network access outside the Gigabit LAN, I cannot use Jumbo
> > Frames, which means I need two Ethernet adapters in each machine.
> > (That's not a problem; I can continue to use the current adapter for
> > Internet access and use the new Gigabit adapters for LAN access.)
> Wouldn't PMTU discovery allow the sending host to retransmit at 1500  
> for anything not destined for the local LAN?

Yes, assuming the routers/firewalls along the way don't try to block
the ICMP messages required for PMTU:


Where you'd probably run into a real problem with jumbo frames is if
you tried mixing jumbo and non-jumbo stuff on the same LAN.
Especially if the router itself doesn't support jumbo frames.

Example: I once worked with a LAN that contained both Ethernet and
FDDI systems (bridged together with no router between them).  The
Ethernet stuff, including the router that connected the LAN to the
rest of the network, would come up with the normal 1500 byte MTU.  The
FDDI servers would come up with an MTU of around 4K.  Everything would
be fine for a while, until some external machine connected to a FDDI
server and asked for enough data at once that the server tried to
respond with a packet larger than 1500 bytes.  The bridge had no way
to get the packet to the router over the Ethernet part of the LAN, and
no way to tell the FDDI machine to stop sending them, so it just threw
the packet away.  The FDDI machine would then try to send it again (or
perhaps an even larger packet), and the bridge would keep discarding
them, resulting in a jammed TCP connection.  A quick workaround was to
cap the FDDI MTU at 1500, but this impacted all of the FDDI-to-FDDI
connections as well and cancelled one reason for using FDDI in the
first place.

                                                  -Dave Dodge

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