: Port 3/5 has become dot1q trunk The other side of the link was not configured, but the trunk became active because the default state of the ports on the other side is auto. The command to view trunk status on CatOS is show port trunk: Switch1-Catos# sho port trunk - indicates vtp domain mismatch # - indicates dot1q-all-tagged enabled on the port $ - indicates non-default dot1q-ethertype value Encapsulation Status Native vlan desirable dot1q trunking nonegotiate trunking nonegotiate isl trunking Vlans allowed on trunk 1-4094 1-4094 1-4094 Vlans allowed and active in management domain 1,10,20 Vlans in spanning tree forwarding state and not pruned 1,10,20 The trunks 15/1 and 16/1 shown in this output are internal trunks. On a 6500 switch running CatOS, trunks exist from the supervisors to the multilayer switch feature cards (MSFCs). The MSFCs are known as slot 15 and 16 when two supervisors are installed. To specify which VLANs can traverse a trunk, use the same set trunk command, and append the VLANs you wish to allow. CatOS works a little differently from IOS in that it will not remove all of the active VLANs in favor of ones you specify: Switch-2# (enable) set trunk 3/5 100 Vlan(s) 100 already allowed on the trunk Please use the 'clear trunk command to remove vlans from allowed list. Configuring Trunks Remember that all VLANs are allowed by default. Preventing a single VLAN from using a trunk is as simple as using the clear trunk command: Switch-2# (enable) clear trunk 3/5 100 Removing Vlan(s) 100 from allowed list. Port 3/5 allowed vlans modified to 1-99,101-4094. You don't have to do a show trunk command to see what VLANs are allowed, because the clear trunk tells you the new status of the port. To limit a CatOS switch SO that only one VLAN is allowed, disallow all the remain- ing VLANs. Just as you removed one VLAN with the clear trunk command, you can remove all of the VLANs except the one you want to allow: Switch-2# (enable) clear trunk 3/5 1-99,101-4094 Removing Vlan(s) 1-99,101-4094 from allowed list. P