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Monday, January 11, 2010

Day 21: Switch Troubleshooting

I had to take a few days off but we're back and today's topic is basic switch troubleshooting.
Basic troubleshooting starts at Layer 1 with switch hardware issues and continue to Layer 2 with possible switch softwareor configuration issues. Only last week I encountered a wrong labeling of an old wall jack! The LED indicators are really helpful, so use them.

There's a variety of available show commands:
  • show running-config: running configuration stored in RAM on the switch
  • show startup-config: startup configuration stored in NVRAM on the switch
  • show version: Cisco IOS software version, image name, memory and processor on a switch
  • show interfaces: includes addressing and security
  • show mac-address-table
  • show port-security
Besides these we also have the Cisco Discovery Protocol. CDP can verify Layer 2 connectivity even when a Layer 3 IP address is not properly configured. It can also be used to learn about the hardware and software configuration of connected devices with CDP enabled.
  • no dcp run:  will disable CDP globally on a switch (from global configuration mode)
  • no cdp enable: will disable CDP on a specific interface (from interface configuration mode)
  • show cdp: shows whether its running on a switch, no info about connected neighbors
  • show cdp neighbors: uses Layer 2 CDP communication to discover and display information about directly connected Cisco devices and their platforms
  • show cdp neighbors detail: this includes the Layer 3 IP address

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