Content

| 4.1 |  | Characteristics of Data Link Layer Problems |
| 4.1.3 |  | The network is functional, but operating at less than the baseline level |

While complete data-link layer failures are relatively easy to identify and rectify, it is much more difficult to isolate problems caused by suboptimal Layer 2 operations. The fact that some, or all, frames do actually reach their destination can make it difficult to determine the boundary of the problem and focus on the devices or link causing the problem. There are two distinct types of suboptimal Layer 2 operation that can occur in a network:

- Frames take an illogical path to their destination, but do arrive.
- Some frames are dropped.

Although these are very different problems that can be caused by Layer 2 or Layer 1 faults or design issues, they share some similar symptom:

- Network delays.
- Lack of throughput.
- Poor network application performance.

It is important to remember that the loss of some frames can be tolerated by many applications, since TCP will retransmit the lost segment. For delay sensitive traffic such as voice, video, or SNA of IBM, suboptimal levels of performance can actually bring the application to a halt.

An example of a problem which would cause frames to take an illogical path through the network would be a poorly designed Layer 2 spanning-tree topology. In this case, the network might experience high bandwidth utilization on links that should not have that level of traffic. Problems that cause frames to actually be dropped can be identified through error counter statistics and console error messages that appear on the switch or router. In an Ethernet environment, an extended or continuous ping will also reveal if any frames are being dropped.

There are Layer 2 protocols, such as X.25, that have their own retransmission mechanisms which will ensure that dropped frames are retransmitted. In this case, it can be difficult to determine if frames are being dropped. Variations in round trip times for ping replies can indicate a problem, but to be certain that frames are being lost or corrupted, an examination of the error counters on the interface would be required.