Which command should be used to verify that a VLAN is defined on a switch to troubleshoot a VLAN forwarding issue?

A. Switch#show interfaces fast 0/0 switchport
B. Switch#show vlan
C. Switch#show vlans
D. Switch#show vtp

B. The first command used to diagnose a VLAN forwarding issue is the show vlan command. This command will show all of the VLANs that are defined on the switch either manually or dynamically through the VLAN Trunking Protocol (VTP).