High CRC Error Count on Interface — Layer 1 or Duplex Issue
Presenting Symptom
High CRC error count observed on an interface, causing packet loss and performance degradation.
Network Context
A small branch office with a Cisco 2960 switch connecting to an ISP router via a copper Ethernet link. The switch runs IOS 15.2. The interface shows increasing CRC errors, impacting user connectivity.
Diagnostic Steps
Check interface status and error counters
show interfaces GigabitEthernet0/1GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
input flow-control is off, output flow-control is unsupported
ARP type: ARPA, ARP Timeout 04:00:00
Last input 00:00:00, output 00:00:00, output hang never
Last clearing of "show interface" counters never
Input queue: 0/2000/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 1000 bits/sec, 1 packets/sec
5 minute output rate 2000 bits/sec, 2 packets/sec
12345 packets input, 1234567 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
1000 input errors, 1000 CRC, 0 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
12345 packets output, 2345678 bytes, 0 underruns
0 output errors, 0 collisions, 0 interface resets
0 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 outHigh CRC errors (1000) indicate Layer 1 issues. Check duplex and speed settings; mismatched duplex can cause CRC errors.
Verify duplex and speed settings on both ends
show interfaces GigabitEthernet0/1 | include duplexFull-duplex, 1000Mb/s, media type is 10/100/1000BaseTX
If the interface shows half-duplex or speed mismatch, that is a common cause. Also check the connected device's settings.
Check for excessive collisions or late collisions
show interfaces GigabitEthernet0/1 | include collisions 0 output errors, 0 collisions, 0 interface resets
0 babbles, 0 late collision, 0 deferredLate collisions indicate duplex mismatch. If late collisions are present, duplex mismatch is likely.
Check the remote device's interface settings (if accessible)
show interfaces GigabitEthernet0/1 | include duplexFull-duplex, 1000Mb/s
Compare with local settings. If remote is half-duplex, mismatch is confirmed.
Root Cause
Duplex mismatch between the Cisco switch and the ISP router. The switch is set to full-duplex while the router is set to half-duplex, causing CRC errors and packet loss.
Resolution
Verification
show interfaces GigabitEthernet0/1 | include CRC Expected output: 0 CRC Also check: show interfaces GigabitEthernet0/1 | include duplex Expected: Full-duplex, 1000Mb/s
Prevention
1. Always use auto-negotiation for both speed and duplex on copper Ethernet links to avoid mismatches. 2. If manual settings are required, ensure both ends are configured identically. 3. Regularly monitor interface error counters using SNMP or syslog.
CCNA Exam Relevance
On CCNA 200-301, this scenario appears in troubleshooting questions where CRC errors indicate Layer 1 or duplex issues. Candidates must interpret show interfaces output and identify duplex mismatch as a common cause. The exam tests the ability to differentiate between cable faults and configuration mismatches.
Exam Tips
Remember that CRC errors often point to Layer 1 issues, but duplex mismatch is a common cause.
Late collisions in the output are a strong indicator of duplex mismatch.
Know that auto-negotiation is recommended; manual settings must match exactly.
Commands Used in This Scenario
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