Interface Errors Explained

“show interface” is one of the more common commands that every network engineer uses. However, sometimes it’s not always clear what some of the displayed interface-level errors mean. This article explains some of the more common errors, their meaning, and possible causes.

  • SymbolErrors
    * device receives invalid symbols in the frame
    * points to physical problems
  • Alignment Errors – both conditions must be met:
    * The number of bits received is an odd byte count
    * The frame has a Frame Check Sequence (FCS) error
    * points to MAC layer or physical problems
  • FCS Errors = frames failing FCS check
    * happens when devices are restarted, or due to line noise (less common)
    * very small number is acceptable
  • CRC Errors
    * an RMON statistic that combines Alignment and FCS errors
  • inDiscards
    * received packets could not be delivered to a higher up protocols b/c of congestion or packet errors
  • outDiscards
    * packets were not transmitted because of network congestion
  • giantFrames
    *packets longer than 1518 bytes, but otherwise OK
  • runtFrames
    *packets smaller than 64 bytes
  • lateCollisions
    *collisions not detected right away most likely due to excess cabling distance
  • deferredTransmissions
    *number of packets that had to wait to be transmitted b/c of busy interface
    *depending on the buffer status these packets may or may not be dropped