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User experiences FCS-err counter increments on a port due to a duplex configuration or hardware problem

Core issue

This issue is caused by a bad cable, Network Interface Card (NIC) or duplex configuration.

The show port counters mod#/port# command shows Frame Check Sequence (FCS)-errors incrementing on a port.

FCS errors can be caused by any of these options:

  • Collisions
  • Duplex mismatch
  • Hardware (NIC, cable or port)
  • Frames with bad FCS being generated by the connected device


To resolve this issue, perform these steps:

  1. If possible, hard set the speed and duplex settings on both the port and the connected device.

    On the switch, issue the set port speed mod#/port# speed and set port duplex mod#/port# duplex commands.

    For example, to set the speed to 100 and the duplex to full on port 5 of module 4, the commands would be as shown:

    set port speed 4/5 100
    set port duplex 4/5 full
  2. Check the cable with a tester or replace it with a known good cable.

    If the connection is to a patch panel, bypass the patch panel and connect directly to the end device.

  3. Check the NIC drivers on the end device to make sure they support the required duplex and speed settings.

    For compatibility information, refer to Troubleshooting Cisco Catalyst Switches to NIC Compatibility Issues.

  4. Check the port by connecting the end device to a different port on the switch.

    Monitor the port counters to see if the FCS-err count is incrementing.

Problem frequency


Device connected to switch



Another Switch

External router

Non Cisco

Show port errors


Show interface counters


New Member

Thanks! This helped, we tried new cable, new drivers, new NIC..turned out to be a bad port. Moved to another port, issue went away.

New Member

Hello Experts,

From last One week I am fully facing the issue with INPUT errors, CRC and Frames. Please find the below details. That is affected on my job, even lot of times I am checking the pots and cables and duplex, speed but continuously errors are increasing.I am only the person of in my organization, Please help me to find out the problem.
Now I am using CISCO 2911 router, every interfaces getting the errors like that only

Router#sh int g0/1
GigabitEthernet0/1 is up, line protocol is up
  Hardware is CN Gigabit Ethernet, address is 00c8.8b61.5371 (bia 00c8.8b61.5371)
  Internet address is
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
     reliability 255/255, txload 2/255, rxload 1/255
  Encapsulation ARPA, loopback not set
  Keepalive set (10 sec)
  Full Duplex, 100Mbps, media type is RJ45
  output flow-control is unsupported, input flow-control is unsupported
  ARP type: ARPA, ARP Timeout 04:00:00
  Last input 00:00:02, output 00:00:00, output hang never
  Last clearing of "show interface" counters 01:32:49
  Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
  Queueing strategy: fifo
  Output queue: 0/40 (size/max)
  5 minute input rate 102000 bits/sec, 59 packets/sec
  5 minute output rate 904000 bits/sec, 97 packets/sec
     232879 packets input, 50659461 bytes, 0 no buffer
     Received 1833 broadcasts (0 IP multicasts)
     0 runts, 0 giants, 0 throttles
     14546 input errors, 7274 CRC, 6472 frame, 0 overrun, 0 ignored
     0 watchdog, 288 multicast, 0 pause input
     343302 packets output, 368712945 bytes, 0 underruns
     0 output errors, 0 collisions, 0 interface resets
     185 unknown protocol drops
     0 babbles, 0 late collision, 0 deferred
     0 lost carrier, 0 no carrier, 0 pause output
     0 output buffer failures, 0 output buffers swapped out

New Member

Assuming you have checked everything in this thread already, are you sure you are not maxing out your router's capacity?

Check this out:


New Member

Thank you SIR,

Basically I don't know, how I will check the maxing out the router's capacity?

Please help me to sort out this issue