Speed/Duplex Mismatch Causing CRC Errors and Low Throughput
Presenting Symptom
Users report slow file transfers and intermittent connectivity; the interface shows increasing CRC errors and runts.
Network Context
A small branch office connects to the corporate WAN via a Cisco 4331 router with a FastEthernet interface (Fa0/0) to a managed switch. The router runs IOS XE 16.9, and the switch is a Cisco Catalyst 2960. The link is configured with default speed/duplex settings, but the switch port is set to 100/full while the router auto-negotiates to 100/half.
Diagnostic Steps
Check interface status and errors
show interfaces fa0/0FastEthernet0/0 is up, line protocol is up
Hardware is Gt96k FE, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
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)
Half-duplex, 100Mb/s
input errors 150, CRC 120, frame 0, overrun 0, ignored 0
0 input packets with dribble condition detected
output errors 0, collisions 0, late collisions 0The interface shows half-duplex and 100Mb/s, with CRC errors and input errors. This indicates a speed/duplex mismatch: the router is operating at half-duplex while the switch is likely set to full-duplex. CRC errors and runts are typical of duplex mismatch.
Verify duplex setting on the switch side
show interfaces gigabitethernet0/1 (on the switch)GigabitEthernet0/1 is up, line protocol is up
Hardware is Gigabit Ethernet, address is 1111.2222.3333 (bia 1111.2222.3333)
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 10/100/1000BaseTX
input errors 0, CRC 0, frame 0, overrun 0, ignored 0
0 input packets with dribble condition detected
output errors 0, collisions 0, late collisions 0The switch port is configured for full-duplex and 100Mb/s. The mismatch is confirmed: router is half-duplex, switch is full-duplex. This causes collisions and CRC errors on the router side.
Check auto-negotiation status on the router
show controllers fa0/0Interface FastEthernet0/0 Hardware is Gt96k FE Media type is 10/100BaseTX Auto-negotiation is enabled ...
Auto-negotiation is enabled on the router, but it negotiated to half-duplex. This often happens when the switch has hard-coded speed/duplex, forcing the router to half-duplex due to the IEEE standard.
Check switch port configuration
show running-config interface gigabitethernet0/1 (on the switch)interface GigabitEthernet0/1 switchport mode access speed 100 duplex full !
The switch port has speed and duplex hard-coded to 100/full. This prevents proper auto-negotiation, causing the router to fall back to half-duplex.
Root Cause
The switch port is manually configured with 'speed 100' and 'duplex full', while the router interface uses default auto-negotiation. When one side is hard-coded, auto-negotiation fails, and the router defaults to half-duplex. This duplex mismatch causes CRC errors, runts, and poor throughput due to collisions and retransmissions.
Resolution
Verification
After applying the fix, verify on the router: show interfaces fa0/0 Expected output: FastEthernet0/0 is up, line protocol is up Full-duplex, 100Mb/s input errors 0, CRC 0, frame 0, overrun 0, ignored 0 output errors 0, collisions 0, late collisions 0 Also verify on the switch: show interfaces gigabitethernet0/1 Expected output: Full-duplex, 100Mb/s input errors 0, CRC 0
Prevention
1. Always use auto-negotiation on both ends for Ethernet links (IEEE standard). 2. If hard-coding is necessary (e.g., for legacy devices), ensure both sides are configured identically. 3. Monitor interface errors regularly; CRC errors often indicate duplex mismatch.
CCNA Exam Relevance
On the CCNA 200-301 exam, speed/duplex mismatch appears in troubleshooting scenarios, often as a drag-and-drop or multiple-choice question. The exam tests the understanding that a duplex mismatch causes CRC errors and poor performance, and that auto-negotiation should be used unless both sides are hard-coded identically.
Exam Tips
Remember: A duplex mismatch always results in CRC errors on the half-duplex side and late collisions on the full-duplex side.
The exam may show 'show interfaces' output with CRC errors and ask for the root cause; look for duplex mismatch.
Know that hard-coding speed/duplex on one side disables auto-negotiation; the other side may fall back to half-duplex.
Commands Used in This Scenario
show controllers
Displays detailed hardware and interface controller status information, used to diagnose physical layer issues such as cable faults, clocking problems, or interface errors.
show interfaces
Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.
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