Troubleshooting
Problem
This document will describe the methods used to determine the source of frames with CRC (cyclic redundancy check) errors, or bit errors from external devices. This document does not cover methods to determine if CRC errors are sourced within a switch, which is a rare condition. The net result of a frame with a CRC error will typically cause the frame to be discarded, either within the switch itself or by the destination device. Regardless of where a CRC frame is discarded, the result is an IO operation will have to be retried which causes some degree of negative impact on performance.
Resolving The Problem
This document assumes the reader does not necessarily have access to various management applications, but does have command line interface access to the switch. Once the root source of CRCs has been determined, the means to correct this situation will be provided in another document titled “How to resolve CRC errors”.
The method to determine the source of frames with CRC errors on Cisco switches is simple and somewhat easier to identify when compared to the methods used with Brocade switches. Cisco switches have the operational characteristic in which the switch will discard a frame detected to have a CRC error at the point of error detection. For external facing switch ports, this means that frames with CRCs are detected, and discarded, by the switch port’s receiver.
There are two commands available to find the source of frames with CRCs. The commands are:
· show interface
· show interface counters
The output of both commands is reasonably similar to each other. For illustration purposes, the following example is from the show interface counters command.
show interface counter
fc1/1
5 minutes input rate 8888736 bits/sec, 1111092 bytes/sec, 624 frames/sec
5 minutes output rate 2855360 bits/sec, 356920 bytes/sec, 400 frames/sec
424107878 frames input, 642308273224 bytes
0 class-2 frames, 0 bytes
424107878 class-3 frames, 642308273224 bytes
0 class-f frames, 0 bytes
0 discards, 0 errors, 36 CRC/FCS
0 unknown class, 0 too long, 0 too short
471089499 frames output, 574867395548 bytes
0 class-2 frames, 0 bytes
471089499 class-3 frames, 574867395548 bytes
0 class-f frames, 0 bytes
0 discards, 0 errors
<output has been truncated>
In this example, port fc1/1 has received 36 frames which contain a CRC error which was the point where the frames were discarded. Since frames with CRC errors are discarded on the ingress side of a port, notice that the output information does not have a counter for frames with CRCs. In this example the device attached to port fc1/1 or the pathway from the attached device port is the root source of the CRC errors. This could be a device such as a host or storage system port or it could be another switch.
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Document Information
Modified date:
17 June 2018
UID
ssg1S1009264