If you search the internet you will find many documents that insist that for jumbo frame to work the entire network must support it - I believe this to be a misunderstanding of a much simpler statement - for jumbo frame to work, the entire network path linking the two devices
must support it.
Ascii graphics time
the (j) indicates that jumbo frame is supported, a doubled line represents gigabit (with or without jumbo frame and a single line represents 100 mbps
You can use jumbo frame between host1 and host 2 provided switch1 supports jumbo frame - host3, host4 & switch2 do not need to support jumbo frame, and you can communicate just fine between host1 and host3, despite the fact that one is gigabit with jumbo frame and the other one is 100 mbps.
I've added a router and another switch here - now let's assume switch 3 does not support jumbo frame but host4 does and you configure it for jumbo frame - you will find that you can communicate between host 3 (which is 100 mbps) and any of the other hosts, you can communicate between host 1 & host 2, but not between host1 & host4 or host2 & host4.
What will happen here is that the hosts negotiate the MSS between them (the switch does not participate in the negotiaition) and agree to a jumbo frame of a particular size which they both support, but, if the network path linking the two cannot accomodate the frame size, the packets will be dropped.
My network actually looks like this
DNS-323<=========>Netgear GS108T<===========>IBM xSeries 206
Netgear FVS124G<==> Netgear FS728TS<------------->Linksys WAP54GP
|| | | | |
Dell Optiplex and other client systems mixed gigabit and 100 mbps
The pairs of doubled lines linking the switches are a LAG (just for the heck of it), and jumbo frame is enabled only on the top row (the DNS-323, the GS108T and the IBM server) and everything works great.
It should be noted that some of what I have here is, strictly speaking, not required for what I need to do, but was installed to allow me to do some "lab practicals" - spanning tree, VLANs, LAGs, adapter teaming (the IBM has dual Intel gigabit cards)