CRC errors : Solution

Answer to the CRC error Question
Taken from CERT-642-831
Related Exam: Cisco 642-831 Internet Troubleshooting Support Exam

Choice b is correct. CRC errors indicate a Physical layer issue. Therefore, the bottom-up approach would be the best troubleshooting method.

There are three valid troubleshooting approaches:

1. Bottom-up approach
2. Top-down approach
3. Divide and conquer approach

Bottom-up - You should use this approach when you know or suspect that the problem is in the Network, Data Link, or Physical layer of the OSI model. Using this approach the troubleshooter begins troubleshooting at the Physical layer and works his or her way up the OSI model. Most network problems reside at the lower levels; therefore, implementing the bottom-up approach will usually result in problem resolution. The disadvantage to this approach is that it can be difficult to find out which network device is causing the problem. If symptoms of the problem come from the network, the bottom up approach will be more effective.

Top-down - You should use this approach when you know or suspect that the problem resides in the top-half of the OSI model. Using this approach the troubleshooter begins troubleshooting at the Application layer and works down to the lower levels of the OSI model. This approach works best when the troubleshooter believes that the problem is with an application and not the network or network devices. The disadvantage to this approach is that it can be difficult to find out which application is causing the problem. If problems come from the users, the top-down approach will be more effective.

Divide and conquer - Using this approach the troubleshooter begins at a middle layer (Transport, Network, or Data Link) and begins testing in both directions. Once the troubleshooter has identified the direction of the problem, he or she can work in that direction until the cause of the problem is identified. For example, if you begin troubleshooting at the Network layer and everything seems to be working, it is safe to assume that the Data Link and Physical layers are working as well.

References:

Cisco Internetwork Troubleshooting Version 5 - Determining an Effective Troubleshooting Strategy