Symbol errors versus CRC errors

Is anything wrong if we get symbol errors but we don’t get corresponding CRC errors? What is the relation between these errors? Are the mutually defendant or mutually exclusive?

Hello Rushabh

Ethernet uses several types of encoding to place the frame onto the physical layer. Gigabit Ethernet for example, 8B/10B encoding to translate 8 bit data from the MAC sublayer to what is called a 10 bit symbol to send over the physical medium. When a port receives a symbol, it decodes and extracts the 8 bit data from the 10 bit symbol.

A Symbol error means that the interface has detected an undefined or invalid symbol that has been received. You may get symbol errors without getting CRC errors. This is because CRC errors are calculated at layer 2 while symbol errors exist on layer 1. Each type of error detects something different.

Keep in mind that small amounts of symbol errors can be ignored. A large number of symbol errors can indicate a bad device, cable, or hardware.

I hope this has been helpful!


Thank you for that detailed explanation! It indeed cleared man y doubts.

I have a follow up question about this!

Why dont the input errors add up correctly? Here I can see some CRC errors and 0 other input errors whihc don’t get added up to result in the mentioned number of errors! Where are the missing ones?

Broadcast address is
  Address determined by manual configuration
  IP MTU 1500 bytes , BW 10000000 kbit
  Full-duplex, 10Gb/s, auto negotiation: off, uni-link: disabled
  Up 175 days, 1 hours, 23 minutes, 15 seconds
  Loopback Mode : None
  0 link status changes since last clear
  Last clearing of "show interface" counters 20:40:14 ago
  5 seconds input rate 2.08 Mbps (0.0% with framing overhead), 1634 packets/sec
  5 seconds output rate 504 kbps (0.0% with framing overhead), 718 packets/sec
     29940890 packets input, 4898352528 bytes
     Received 0 broadcasts, 7623 multicast
     4 runts, 0 giants
     54 **input errors, 38 CRC, 0 alignment, 0 symbol, 0 input discards**
     0 PAUSE input
     12491385 packets output, 1117699471 bytes
     Sent 0 broadcasts, 2481 multicast
     0 output errors, 0 collisions
     0 late collision, 0 deferred, 0 output discards
     0 PAUSE output

Hello Rushabh.

Input errors are not only limited to the CRC, alignemtn, symbol etc that are shown in the above output. There are also input errors that relate to other things such as frame drops because of congestion, frames that have arrived at the switch with too low of a voltage on the wire to be read or an incorrect preamble and thus an incorrect indication of start of frame. These are not all indicated in the output. Sometime one frame may have two or more errors on it, yet it is still counted as one error, so the numbers don’t always add up.

I hope this has been helpful!