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 (see Chapter 3 for details).
  • 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 fastethernet0/1 switchport command to verify the trunk encapsulation.
  • Use the vtp status command to verify the domain name and revision number.