Troubleshooting Switches

Here are some common troubleshooting techniques to try when configuring VLANs:

[*] Verify physical connections and layer 2 (data link layer).

*Is the cable plugged into the correct port?
*Is the link light on, and if so, what color is it? (green = forwarding, yellow = blocking, blinking yellow = error)
*Use show interface command to verify link state (up or down).
*Verify duplex and speed settings. (Remember that autonegotiation is unreliable.)
*Use CDP to see if the Cisco devices can see each other.
*Verify that VLAN 1 has been configured and that a default gateway has been configured.

[*] If your VLANs can't see each other or your edge routers, verify your router and switch configurations:

*Is the router a member of all VLANs?
*Do you need trunking?
*If you have a router in each VLAN, verify the router's configuration.

[*] Verify VLAN configuration:

*Is the port in the correct VLAN?
*Is there an "allowed" statement in the trunking configuration?
*Use show VLAN.
*Use show interface switchport.
*Use show spanning-tree.

[*] If two switches don't seem to be sharing VLAN information or are not forwarding frames, verify the VTP configuration:

*Is trunking enabled between the two switches?
*Are both switches using the same trunk encapsulation (ISL, dot1q, etc.)?
*Use the show interface fastethernet0/? switchport command to verify the trunk encapsulation.
*Use the show vtp status command to verify the domain name and revision number.


bi0os