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.