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.

Getting Started

IOS Images and Configuration Files

Basic Router Configuration

Line Commands

Interface Commands

Networking Technologies

Access Lists

IP Routing Topics

Interior Routing Protocols

Border Gateway Protocol

Quality of Service

Dial-on-Demand Routing

Specialized Networking Topics

Switches and VLANs

Router Security

Troubleshooting and Logging

Quick Reference

Appendix A Network Basics


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 © 2008-2020.
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