Chapter 3 was also a reasonabl;y long chapter and wrapped up the main switch section of the book. It started with an overall troubleshooting methodology and then got into the specifics. No new commands or concepts were introduced. Instead, it basically covered all the commands from the first two chapters and discussed how to use them for troubleshooting. Therefore, my notes were a bit limited. Here they are:
4 Main Troubleshooting Steps for Switches:
1. Confirm the network diagram using CDP (Cisco Discovery Protocol)
2. Find any interface problems (down/down, up/down, etc.)
3. Find if any filtering or port security issues exist
4. Determine if any VLAN or Trunking issues exist
For interfaces connected in up/up state but still having issues, check for duplex mismatch with show interfaces command. Look for runts, collisions, or late collisions which can point to a duplex mismatch.
Switch# show port-security interface FA0/1
to look for port security issues.
If the show vlan and show interface switchport commands are not available on the exam, use the show mac address-table command to find the VLAN.