Can I use jumbo frames on my LAN?

[Nikolas Britton]

Is it true that jumbo frames are only useable with VLANs or LANs that
are 100% GigE?

The current LAN has a 16port 100Base-TX switch with a mix of 10/100
devices connected to it. 4 of the machines connected to the switch
have built-in GigE support and their usage profile can take full
advantage of it, so I'm going to be adding a 8port GigE switch to the
current setup. The server, 4 workstations, and the old switch will be
connected to this new switch. None of the switches supports 802.1q and
the systems are all on the same subnet anyways.

So if I tell my FreeBSD server that I want to use 9000 byte frames
what happens to my other 10 and 100base-TX systems on the old switch?
Does FreeBSD dynamically change it's mtu size based on the system it's
talking to or will it always speak 9000 bytes at a time?

My educated guess is that it always speaks 9000 bytes at a time and
therefore I can't use jumbo frames on my network, it will screw up my
10 and 100base-TX systems. Am I right?