3com switch with 2.4.17

I have a 486 which runs as a router, and I want to change to 2.4. After boot and working out which order the ether cards are now found in, everything is OK except I can't ping PCs connected to a 3com switch, which connects to a ne2k-pci RTL-8029 card in the 486 via a hub. PCs directly connected to the hub ping OK.
Does this ring any bells as to why this could be OK with a 2.2.13 kernel but broken with 2.4.17?
My troubleshooting is normally a) check it electricaly (obviously OK as it has worked with 2.2.13 for months) then check it pings.
How can I look further into this? eg looking at ethernet packets instead of TCP/IP packets?
Yours
Ian