...
...
Can you, with the patch cable disconnected, ping a VLAN4 host from another VLAN4 host at the same time as you are pinging a VLAN2 host from another VLAN2 host? Set up a continuous ping as described above and then connect the patch cable - what happens?
Are you using the same ip schema on both VLANs? If you're not, ping won't work, you can expect to get errors when you ping with the actual error depending on the configuration of the network AND the hosts.
Before path cable insertion:
VLAN2 (main) can ping other machines on VLAN2 (main) but not machines VLAN4 (test).
VLAN4 (test) can ping machines on VLAN4 (test) but not machines on VLAN2 (main).
They are all doing ping with -t, so it just runs continuously. Once patch cable is inserted, no change, as if it wasn't there.
On the other hand, when VLAN4 (test) machines are on the side switch, VLAN2 and side-switch (test) machines can ping each other no problem as expected. Machines on the side are fix-IP for this test (since unplug and re-plug will cause a PC to do DHCP again, those on the side are not getting DHCP). All machines are 192.168.4.x with 255.255.255.0 mask.
Maybe it has some mechanism to prevent establishing link with itself. What if you join the VLANs through an intermediate switch?
(Kinda doesn't solve the problem of not wanting to have another switch around, I know, just a check ).
What's on the link status indicators and all that stuff?
Done that (patch via switch) before and again today, as you say, just to check. No difference.
Status light on the switch just show up/down and speed. Nothing interesting.
***
I accept that the 3com switch probably has some code to prevent one port connected to another port on itself. But it does so without regards to VLAN. Annoying, but so it goes.
Thanks for all the input. Good for my learning experience.