Troubleshoot VLANs and Trunks
November 11, 2016
This post lists the commands useful for troubleshooting VLAN and Trunk ports on Cisco switch.
SW1# show vlan brief // list each vlan and its interfaces; doesn't list operational trunk SW1# show vlan // list each vlan and its interfaces; doesn't list operational trunk SW1# show vlan id NUM // list access ports and trunk ports which allow VLAN SW1# show interface switchport SW1# show interfaces gigabit0/2 switchport // identifies the interface's access VLAN, voice VLAN and operational mode (access/trunk) SW1# show interfaces trunk // shows also information about vlans blocked by STP on specyfic trunk interface; shows info only about currently operational trunks SW1# show interfaces fa0/11 status // shows in which vlan is interface or "trunk" information SW1# debug dtp packets // debug DTP process
Configure your port explicitly as trunk or access port – it is more more secure and predictive.
Switch(config-if)#switchport mode access // this command also disables DTP Switch(config-if)#switchport mode trunk // DTP still works Switch(config-if)#switchport nonegotiate // explicitly disable DTP