I have a 3com Unified switch and a 4500. On the Unified switch I had configured a VLAN 2 on ports 21-24. The remaining 20 ports were on the default VLAN 1 and port 1 was plugged into port 1 of my 4500 switch allowing the 47 ports from the 4500 to work with the remaining 19 ports on the Unified. None of the VLAN 1 ports were tagged.
I need to create a VLAN 3 and have ports on both switches to be part of it so I configured VLAN 3 on both switches. I then configured ports 10-15 on both switches to be part of VLAN 3 untagged. I then plugged ports 10 together. The traffic on each switch is isolated from the other vlans and can communication with each other on the same switch, but not across the two switches. VLAN 1 is still fine.
What else needs to be done to get VLAN 3 speak with devices on both switches.