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