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.

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