Chapter 10: Ethernet Switch Troubleshooting

Troubleshooting is very important for a CCNA technician. In this chapter we see tools and methods that will help you identify and solve most network problems. Consider using the techniques in this chapter as a guide to logical problem-solving processes. The steps in this tutorial address computer connection problems in Cisco Command-Line Interface (CLI).

The process of troubleshooting help you the problem or the issue you are having. It makes you think in all the possible causes of it and test your skills as technicians. There are three basic steps in the troubleshooting process: Analyzing/Predicting normal operations-in this step you basically predict what should happen if the network is working correctly, based on documentation, configuration, and show and debug command output; Step 2: Problem isolation-Determine how farer do the packet can go and show and debug command output; Step 3: Root cause analysis- Identify causes of the problems identified and how can be fixed it. The documentation and the show command output are helpful in this process to confirm network topology and predict its normal behavior. Isolating the problem at Layer 3 point out what went wrong in Layer 1 and Layer 2.

The more supportive components in this process are Cisco Discovery Protocol (CDP), Examining interface status and Analyzing where frames will be forwarded. CDP lend a hand to discover and/or confirm documentations by listening for advertisements sent by other devices. It usually discover device identifier, address list, local interface, port identifier, capabilities list, and platform. CDP create a secure explosive to restrict the access but is easy to disable it.

For the next step in our adventures in troubleshooting let’s start with the idea that before a switch can either send or receive data frames, it must be working properly. Switches have two status codes to determine if an interface is working. They are named the line status and protocol status. The line status is use when the layer 1 is working and protocol status is when layer 2 is working. Their values are shown either “up” or “down”. When there are speed and duplex mismatches is exhausting to troubleshoot. Switches and routers can have different speed (10,100 or 1000) and different duplex (full or half) interface. Keep in mind when you configuring both speed and duplex on the switch interface, the IEEE-standard auto-negotiation process will be disabled- deactivating the agreement of fastest speed form both devices. Some common LAN Layer 1 problems indicators are excessive noise, collisions, and late collisions.

Switches are smart devices that learn MAC address and store them to make forwarding/filtering decision for each frame in the future. The show MAC address-table EXEC command displays the content MAC address table, which display the lists all MAC addresses currently known by the switch. The port security feature restrict input to an interface by limiting and identifying MAC addresses of the workstations that are allowed to access the port. When you assign secure MAC addresses to a secure port, the port does not forward packets with source addresses outside the LAN.