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.




Cisco IOS in a Nutshell
Cisco IOS in a Nutshell (In a Nutshell (OReilly))
ISBN: 0596008694
EAN: 2147483647
Year: 2006
Pages: 1031
Authors: James Boney
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