The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

16.5. Some Strategies for Layer 2 Troubleshooting

In keeping with the theme of the book, this section focuses on some Layer 2 troubleshooting considerations. After you have determined that there is not a physical layer issue, the data link layer is where you need to focus. Consider the following questions:

1. Are you seeing errors on the Layer 2 port?

   The port may be receiving or sending any number of errors that will assist in running down the problem:

   • FCS/CRC errors

   • Giant datagrams

   • Runt datagrams

   • Alignment errors

2. What are the speed and duplex settings on each end of the link? Unless there is a specific reason, auto negotiation should be used. Duplex mismatches occur when both ends are not set the same.

3. What type of cabling is being used?

   If the correct cable type is not used, you will have transmission errors.

4. What devices are between the two problem endpoints?

5. What other devices are on the segment?

6. Are you saturating the link with traffic?

7. Are you seeing any congestion on other ports in the VLAN?

8. Are you seeing congestion on uplink ports specifically?

16.5.1. Performing a Health Check

When you are troubleshooting or are just monitoring the bridged catenet, there are some specific things that you can do to check the health of the bridge (or bridges) that you are experiencing problems with.

• Check the CPU utilization. If the CPU is spiking and the spikes are sustained, then you can begin focusing on the processes that are tying up the bridges processor.

• Check device logs. See if there are any ...