commands can be extremely useful when troubleshooting VLANs. Figure illustrates the most common problems found when troubleshooting VLANs.

To troubleshoot the operation of Fast Ethernet router connections to switches, it is necessary to make sure that the router interface configuration is complete and correct. Verify that an IP address is not configured on the Fast Ethernet interface. IP addresses are configured on each subinterface of a VLAN connection. Verify that the duplex configuration on the router matches that on the appropriate port/interface on the switch.

The show vlan command displays the VLAN information on the switch. Figure , displays the output from the show vlan command. The display shows  the VLAN ID, name, status, and assigned ports.

The CatOS show vlan keyword options and keyword syntax descriptions of each field are also shown.

The show vlan displays information about that VLAN on the router. The show vlan command followed by the VLAN number displays specific information about that VLAN on the router. Output from the command includes the VLAN ID, router subinterface, and protocol information.

The show spanning-tree command displays the spanning-tree topology known to the router. This command will show the STP settings used by the router for a spanning-tree bridge in the router and switch network.

The first part of the show spanning-tree output lists global spanning tree configuration parameters, followed by those that are specific to given interfaces.

Bridge Group 1 is executing the IEEE compatible Spanning-Tree Protocol.

The following lines of output show the current operating parameters of the spanning tree:

Bridge Identifier has priority 32768, address 0008.e32e.e600
Configured hello time 2, Max age 20, forward delay 15

The following line of output shows that the router is the root of the spanning tree:

We are the root of the spanning tree.

Key information from the show spanning-tree command creates a map of the STP network.

The debug sw-vlan packets command displays general information about VLAN packets received but not configured to support the router.  VLAN packets that the router is configured to route or switch are counted and indicated when using the show sw-vlan command.