Switchport Errors – Determining the cause

Many times we (network engineers) hear the complaint “the network is working, but it is terribly slow”. This is often one user’s perception of a perfectly working network however, other times there is something to the complaint.  One thing to check is if that particular user’s switchport is reporting any errors.  Let’s take a look at the error counters on a typical switchport.

2940#show int faste0/1 counters errors

Port        Align-Err    FCS-Err   Xmit-Err    Rcv-Err UnderSize
Fa0/1               0          0          0         85         0

Port      Single-Col Multi-Col  Late-Col Excess-Col Carri-Sen     Runts    Giants
Fa0/1              0         0         0          0        16         0         0

When I was a younger engineer I used to look at output like this and have literally no idea what any of it meant.  I just wanted to make sure that there weren’t tons of numbers in the output.  After working in the field for awhile I look at this output differently.  Each type can point you in the direction of what’s causing the error.  Below I’ll list some common causes for each of these error types.  Admittedly I don’t have all of these memorized, but as with all things if you don’t have it memorized, you just have to know where to find it.

Note: These errors can be caused by a variety of things.  I’m only listing the most common issues and solutions I have seen.

Align-Err
Cause: Malformed frames, bad hardware, duplex mismatch, and half duplex collisions
Check: Cabling, physical port, duplex settings

FCS-Err
Cause: Bad frame checksum, duplex mismatch, bad hardware
Check: Cabling, physical port, duplex settings

Xmit-Err
Cause: Speed mismatch
Check: Manual speed defined on a port

Rcv-Err
Cause: Duplex mismatch, backplane congestion
Check: Duplex settings

UnderSize
Cause: Receiving frames that are less than 64 bytes
Check: Device sourcing the frames

Single-Col
Cause: A single collision occurred prior to the switch being able to transmit to media.  Common on half duplex interfaces
Check: Duplex mismatch, high link utilization

Multi-Col
Cause: Multiple collisions occurred prior to the switch being able to transmit to media.  Common on half duplex interfaces
Check: Duplex mismatch, high link utilization

Late-Col
Cause: Collision detected prior to frame fully being transmitted
Check: Cable length, duplex mismatch

Excess-Col
Cause: Switch encountered 16 successive collisions in a row
Check: Duplex mismatch, high segment or link utilization

Carri-Sen
Cause: This error counter is incremented each time the switch tries to send data on a half duple link.  With half duplex the port has to check the wire to ensure its open  prior to sending the frame. 
Check: Duplex settings

Runts
Cause: Frame is too small (less than 64 bytes) and has bad CRC
Check: Duplex mismatch, cabling, physical port

Giants
Cause: Frame exceeds size of 1518 bytes and has a bad FCS
Check: Device sourcing the frames

This post obviously wasn’t meant to tell you absolutely everything about frame and switchport errors.  However, hopefully it has put you on the right track as far as troubleshooting port errors is concerned!