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| 4.1 |  | Characteristics of Data Link Layer Problems |
| 4.1.8 |  | Upper layer component operation |

A failure at the data-link layer will have a profound effect on those layers above it.

A failure at the data-link layer level can be either total or intermittent. A total failure at the data-link layer level will completely prevent communication at higher layers. However, the effects of intermittent errors at the data-link layer will depend on the particular Layer 2 protocol in use. If the Layer 2 protocol is designed to implement reliable communications such as X.25, then it may be difficult to identify Layer 2 problems from the behavior of the upper layers. This is because the Layer 2 protocol will simply retransmit any lost frames. If frames are lost frequently, then application network performance may be sluggish or application time-outs may occur.

For Layer 2 protocols that can be classified as best effort delivery, such as Ethernet, the loss of a frame immediately translates to the loss of a packet and in the case of TCP, a segment. In this case it is the responsibility of TCP to retransmit lost segments. Once again, if the frame losses are intermittent the effects on the application may be that it runs sluggishly or the loss of performance may not be noticeable at all. However, TCP statistics should show evidence of the segment loss and retransmissions.

The chart in Figure outlines the symptoms that might be experienced at each of the layers of the TCP model when an intermittent or total Layer 2 failure occurs.

Although these characteristics give some indication that a Layer 2 problem exists, it will be inconclusive until the devices attached to the non-functional data link are examined. Performing a traceroute or tracert command will immediately identify the failing link, allowing the devices at each end of the link to be checked for appropriate Layer 2 to Layer 3 mapping operations.

The table in Figure summarizes the effect of data-link layer problems on the other layers. Use these characteristics to assist in determining whether a problem being experienced could be a Layer 2 failure.