Roger,
It sounds like it could be a duplex problem. A switch-to-switch connection should be able to run at the fastest speed that the ports have in common, and should be full duplex for maximum performance.
Undersize packets normally come from collisions in the form of fragments (undersize packets with bad CRC); there shouldn't be any collisions on a full duplex connection, though.
It could also be that if the Extreme switch is sending undersize packets with good CRC then maybe the software on the Extreme switch needs updating.
What kind of ports are you using to connect the Cisco Catalyst 3750 to the Extreme Summit7i? What speed and duplex does each side of the link report? Is one side set to autonegotiate, and the other set manually to a fixed speed and/or duplex? What's the distance between the ports? And the media being used: copper UTP, multimode fiber, or single-mode fiber?
If copper (RJ45 connector) on the Extreme switch, it could be to either a Gigabit-only Ethernet GBIC; or built-in autonegotiating 100/1000BASE-T, depending on the model number of the switch. On the Cisco switch, it could be 10/100 or 10/100/1000 depending on the switch model, or 1000-only if you're using the 1000BASE-T SFP.
If fiber (SC or MT-RJ connectors) on the Extreme switch, I don't recall whether their Gigabit-only fiber ports give you the option of what duplex you run. But the Cisco switch's Gigabit-only fiber ports always run in full duplex mode only.
Whatever ports you're using, the actual connection speed and duplex on each end of the switch-to-switch link need to match. Either both ends autonegotiate to matching values; or you need to manually set these values.
Here's a link that might also be useful:
Troubleshooting Switch Port and Interface Problems
http://www.cisco.com/warp/public/473/53.shtml
Hope this helps.
