High Input Errors on Interface — Root Cause and Fix
Presenting Symptom
The network engineer observes that an interface on a Cisco switch shows a high number of input errors, including runts, CRC errors, and frame errors, leading to packet loss and performance degradation.
Network Context
This issue occurs in a small branch office with a Cisco Catalyst 2960 switch running IOS 15.2, connecting end-user devices to the corporate network via a single uplink to a distribution switch. The switch has been in operation for several years, and the problematic interface is a FastEthernet 0/1 connected to a desktop PC.
Diagnostic Steps
Check Interface Status and Error Counters
show interfaces fastEthernet 0/1FastEthernet0/1 is up, line protocol is up
Hardware is Fast Ethernet, address is 0011.2233.4455 (bia 0011.2233.4455)
MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 100Mb/s, media type is 10/100BaseTX
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: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0
Queueing strategy: fifo
Output queue: 0/40 (size/max)
5 minute input rate 1000 bits/sec, 2 packets/sec
5 minute output rate 500 bits/sec, 1 packets/sec
123456 packets input, 12345678 bytes
Received 1234 broadcasts, 0 runts, 0 giants, 0 throttles
1234 input errors, 1234 CRC, 1234 frame, 0 overrun, 0 ignored
0 watchdog, 0 multicast, 0 pause input
0 input packets with dribble condition detected
123456 packets output, 12345678 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 outLook for input errors, CRC, frame errors, and runts. High counts indicate physical layer issues. Also check duplex and speed settings; mismatch can cause errors.
Verify Duplex and Speed Settings
show interfaces fastEthernet 0/1 | include duplex|speedFull-duplex, 100Mb/s
If the interface shows half-duplex or speed mismatch with the connected device, that is a common cause of input errors. The expected output should match the settings on the connected device.
Check the Connected Device's Interface Settings
From the connected PC or device, check its NIC settings (e.g., using ethtool on Linux or adapter settings on Windows). Alternatively, if the connected device is another Cisco switch, use 'show interfaces' on that device.For a PC: typically shows Auto-Negotiation enabled, 100Mbps, Full Duplex. For a switch: similar output as above.
Compare the duplex and speed settings. If one side is hardcoded and the other is auto-negotiating, a mismatch occurs. For example, if the switch is hardcoded to 100/full and the PC is auto, the PC may negotiate to 100/half, causing errors.
Check for Faulty Cabling or Hardware
show interfaces fastEthernet 0/1 | include CRC|frame|runts1234 input errors, 1234 CRC, 1234 frame, 0 runts
High CRC and frame errors often indicate cabling issues (e.g., bad cable, excessive length, interference) or a faulty port. Runts can indicate collisions or duplex mismatch. If errors increase with traffic, suspect physical layer.
Clear Counters and Monitor
clear counters fastEthernet 0/1Clearing counters on interface FastEthernet0/1...
After clearing, monitor the interface for new errors. If errors reappear quickly, the problem is ongoing. If no new errors, the issue might have been transient.
Root Cause
The root cause is a duplex mismatch: the switch port is hardcoded to 100/full, while the connected PC is set to auto-negotiate. The PC auto-negotiates to 100/half because the switch does not advertise full duplex during negotiation. This mismatch causes collisions, CRC errors, and frame errors on the switch interface.
Resolution
Verification
After applying the fix, verify with: show interfaces fastEthernet 0/1 | include duplex|speed|input errors Expected output: Full-duplex, 100Mb/s 0 input errors, 0 CRC, 0 frame, 0 runts Also check that the interface is up/up and no new errors appear after traffic.
Prevention
1. Always use auto-negotiation on both ends of a link unless there is a specific reason to hardcode (e.g., legacy devices that do not support auto-negotiation). 2. Use consistent cabling standards (e.g., Cat5e or better for 100Mbps). 3. Regularly monitor interface error counters using SNMP or syslog to detect issues early.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario appears in troubleshooting questions, often as a multiple-choice or drag-and-drop where you must identify the cause of input errors. The exam tests understanding of duplex mismatch as a common cause of CRC and frame errors. Key fact: A duplex mismatch causes collisions, runts, and CRC errors on the full-duplex side.
Exam Tips
Memorize that a duplex mismatch results in high input errors (CRC, frame) on the full-duplex side and late collisions on the half-duplex side.
Remember that 'show interfaces' output includes error counters; focus on input errors, CRC, and frame errors for physical layer issues.
On the exam, if you see input errors but no output errors, suspect a duplex mismatch or cabling issue, not a faulty switch port.
Commands Used in This Scenario
clear counters
Resets interface counters (e.g., input/output errors, packets) to zero, typically used to clear statistics before monitoring or troubleshooting.
show interfaces
Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.
Test Your CCNA Knowledge
Practice with scenario-based questions to prepare for the CCNA 200-301 exam.
Practice CCNA Questions