Troubleshoot VLANs and Trunks

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

Tips

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

Leave a Reply

Your email address will not be published. Required fields are marked *