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| 4.1 |  | Characteristics of Data Link Layer Problems |
| 4.1.4 |  | Framing errors |

The data-link layer uses standardized frame formats and physical addressing to establish communications within a broadcast domain. If devices disagree on the frame format communications will fail.

Problems with the frame format usually translate to encapsulation incompatibilities between nodes. Actual framing errors occur when a frame does not end on an 8-bit byte boundary for one of the following reasons:

- Noisy serial line.
- Improperly designed cable. This could be caused by serial cable that is too long, or the cable from the CSU or DSU to the router is not shielded.
- The CSU line clock is incorrectly configured. For example, the clock on one of the CSUs may be configured for local clocking.
- Ones density problem on T1 link. This is caused by incorrect framing or a coding specification.

All of these problems result in a receiver having difficulty establishing where one frame ends and another frame starts. When the interface is capable of recognizing this condition, the show interfaces command will reveal an incrementing frame error count.

These problems are critical as they will cause the communications protocol itself to fail or frames will be incorrectly addressed and fail to reach their destination.

At times the framing problems may be minor and the interface may not recognize that a framing error has occurred. However, a misread bit will result in the Layer 2 frame having an invalid Cyclic Redundancy Check or CRC. This error will also be seen as an incrementing error count with the show interfaces command. In this case the CRC error count will be incrementing.

Depending on the severity of the framing problem, the interface may be able to interpret some of the frames. Too many invalid frames may prevent valid keepalives from being exchanged, causing the show interfaces command to report:

> Interface is up, line protocol is down

In summary, the following are all symptoms of a framing or related clocking problem:

- Frame errors
- CRC errors
- Interface and/or line protocol down