An interface shows CRC errors that keep climbing. What does it mean and what do you do?
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A CRC error is a frame whose frame check sequence did not match: the bits were damaged in transit. It is counted on the receiving side, so the cause is the cable, connector, optic or the far end's transmitter feeding this port, or a duplex mismatch. I confirm it is still climbing, check light levels on both ends, look at the far side, then clean or replace the patch lead, optic or port one at a time and clear counters to prove the fix.
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What the counter means
Ethernet's 4-byte FCS is a CRC-32 over the frame. The receiver recomputes it, and on a mismatch it drops the frame and counts CRC and input errors. The sender's counters stay clean, so the side reporting CRC is the receiver of the bad direction.
Likely causes, most common first
- Dirty or damaged fiber connector, bad patch lead, a bend that is too tight.
- A failing or wrong optic, or low receive power. Check
show interfaces transceiver detail. - Copper: damaged cable, poor termination, a run beyond 100 m, interference near power cabling.
- Duplex mismatch: the full-duplex side counts CRC and runts.
- A faulty port or NIC at the far end.
Cut-through switches
A cut-through switch starts forwarding before it has the FCS, so it passes a corrupt frame on and the next device counts the error. On cut-through fabrics, find the first port that counts the frame as received with errors. Some platforms count forwarded bad frames separately; Nexus calls them stomped CRC.
What I do
- Confirm it is active: note the count and check again, or
clear countersand watch. - Check transceiver light levels on both ends of a fiber link.
- Check the far end's counters for the reverse direction.
- Change one thing at a time: clean and reseat, swap the patch lead, swap the optic, move ports.
- Clear counters and confirm CRC stays at zero under real load.
A healthy modern link runs with essentially zero CRC errors. Any steady increase is a fault.
They will ask next
The CRC count is high but not increasing. Do you care?
Check when the counters were last cleared. A historic count from a lead replaced months ago does not matter. Clear the counters and look for new errors.
What do CRC errors do to TCP traffic?
The damaged frame is dropped, so TCP sees loss, retransmits and cuts its sending rate. Even 0.1 percent loss noticeably lowers throughput on a long-RTT path.
Traps
- Looking for the fault on the sending side's counters. The receiver counts CRC.
- Replacing the optic, the lead and the port at once, so nobody knows which part was bad.