3Com 7757 Configuration Manual page 119

Isolate-User-VLAN Configuration Example 119
[SwitchC-Ethernet2/0/4] quit
[SwitchC] interface Ethernet 2/0/1
[SwitchC-Ethernet2/0/1] port link-type hybrid
[SwitchC-Ethernet2/0/1] port hybrid vlan 3 untagged
[SwitchC-Ethernet2/0/1] port hybrid vlan 4 untagged
[SwitchC-Ethernet2/0/1] port hybrid vlan 6 untagged
[SwitchC-Ethernet2/0/1] port hybrid pvid vlan 6
[SwitchB-Ethernet2/0/1] undo port hybrid vlan 1
# Configure isolate-user-VLAN-to-secondary VLAN mapping.
[SwitchC-Ethernet2/0/1] quit
[SwitchC] isolate-user-vlan 6 secondary 3 to 4
After the above configurations, Switch A can receive packets from Switch B and
Switch C, and they are all packets without VLAN tag. Each VLAN 3 configured on
Switch B and Switch C cannot communicate with each other because the packets
from them are stripped off the original VLAN tags before reaching Switch A and
then be encapsulated with the VLAN tag set on Switch A. This makes the lower
switches only own locally valid VLAN configuration. And in this way, the global
VLAN resource is saved.