When a problem is suspected with a switch configuration, use the various verification commands to examine the configuration and identify the problem.
The screen output in Figure 1 shows the results of the show interfaces interface-id switchport command. Assume that you have issued these commands because you suspect that VLAN 10 has not been assigned to port F0/4 on switch S1. The top highlighted area shows that port F0/4 on switch S1 is in access mode, but it does not show that it has been directly assigned to VLAN 10. The bottom highlighted area confirms that port F0/4 is still set to the default VLAN. The show running-config and the show interface interface-id switchport commands are useful for identifying VLAN assignment and port configuration issues.
Figure 2 shows that after a device configuration has changed, communication between router R1 and switch S1 has stopped. The link between the router and the switch is supposed to be a trunk link. The screen output shows the results of the show interface interface_id switchport and the show running-config commands. The top highlighted area confirms that port F0/4 on switch S1 is in access mode, not trunk mode. The bottom highlighted area also confirms that port F0/4 has been configured for access mode.