Topic 1.4 – Identify Interface and Cable Issues
(Collisions, Errors, Duplex Mismatch, and Speed Issues)
This topic focuses on how to detect and troubleshoot physical and data link layer problems that affect network communication between devices such as switches, routers, and computers.
In real IT environments, these issues can cause slow performance, dropped packets, or no connectivity.
1️⃣ Understanding Interface and Cable Issues
Every network device (switch, router, PC, server) uses interfaces (ports) to connect using network cables.
Each interface has settings and statistics that tell you how well communication is happening — such as speed, duplex, and errors.
When there are problems with the cable or interface configuration, you’ll see error counters increase on the interface.
You can view these details on Cisco devices using:
show interfaces
or
show interfaces <interface-name>
2️⃣ Collisions
What are Collisions?
A collision happens when two devices transmit data at the same time on the same shared network segment.
This causes the data to interfere with each other, and both devices must resend the data.
Collisions were common in half-duplex Ethernet networks (like old hubs).
In modern full-duplex switched networks, collisions should not occur.
If collisions are seen today, it usually means something is wrong — such as a duplex mismatch or bad cable.
Where You See It:
In Cisco IOS:
show interfaces
Look for counters like:
- collisions
- late collisions
Symptoms:
- Slow network performance
- Retransmissions
- Increased delay
Causes:
- Duplex mismatch
- Bad cabling
- Faulty network interface card (NIC)
- Using old hubs (shared media)
3️⃣ Errors
Errors indicate problems in transmitting or receiving data over a network interface.
They usually mean that frames or bits were damaged during transmission.
Common Types of Errors (as seen in show interfaces):
| Error Type | Meaning | Common Cause |
|---|---|---|
| Input errors | Total number of errors while receiving frames | Bad cable, interference, or duplex mismatch |
| CRC errors | Frame received but the Cyclic Redundancy Check failed (data corrupted) | Faulty cable, EMI (electrical interference), or duplex mismatch |
| Runts | Frames smaller than 64 bytes | Collisions or faulty NIC |
| Giants | Frames larger than 1518 bytes | Configuration errors or faulty hardware |
| Output errors | Errors while sending frames | Bad cable or hardware |
| Frame errors | Frames received with alignment or format issues | Cabling or speed mismatch |
How to Check:
show interfaces gigabitEthernet0/1
You might see:
Input errors: 25, CRC: 10, Frame: 5, Overruns: 0, Ignored: 0
If these numbers keep increasing, it means there’s a physical or configuration issue.
4️⃣ Duplex Mismatch
What is Duplex?
Duplex defines how communication happens between two devices:
| Mode | Description |
|---|---|
| Half-duplex | Data can flow in one direction at a time (send or receive) |
| Full-duplex | Data can flow in both directions at the same time |
Modern Ethernet (Fast Ethernet, Gigabit Ethernet, etc.) normally runs in full-duplex.
What is Duplex Mismatch?
A duplex mismatch happens when one device is set to full-duplex and the other is set to half-duplex.
Example:
- Switch port: Full-duplex
- PC NIC: Half-duplex
Result:
- Collisions
- Late collisions
- CRC errors
- Slow performance
- High retransmissions
How It Happens:
If one device is set to auto-negotiate and the other is manually configured, auto-negotiation may fail, leading to mismatch.
How to Check:
show interfaces gigabitEthernet0/1
Look for:
Full-duplex, 100Mb/s
If two connected devices show different duplex modes, there’s a mismatch.
How to Fix:
- Configure both sides to auto-negotiate, or
- Manually set both sides to the same speed and duplex
Example:
interface gigabitEthernet0/1
speed 100
duplex full
5️⃣ Speed Mismatch
What is Speed Mismatch?
A speed mismatch happens when the two connected interfaces run at different speeds (e.g., one at 100 Mbps and the other at 1 Gbps).
Result:
- Link might not come up
- Interface might stay down/down
- Poor or no communication
How It Happens:
- Manual misconfiguration
- Old cables not supporting higher speeds
- Faulty or damaged transceivers
How to Check:
show interfaces gigabitEthernet0/1
Example output:
Full-duplex, 100Mb/s
Check both sides of the connection — they must match.
How to Fix:
- Set both sides to auto speed negotiation
- Or manually set the same speed on both sides
Example:
interface gigabitEthernet0/1
speed 1000
duplex full
6️⃣ Cable Issues
Common Physical Cable Problems:
| Issue | Cause | Effect |
|---|---|---|
| Bad cable / loose connector | Poor quality or damaged cable | CRC and input errors |
| Wrong cable type | Using crossover instead of straight-through (or vice versa) | No link or unstable connection |
| Exceeding cable length | Cable longer than Ethernet standard (e.g., >100m for Cat5e) | Interference, packet loss |
| EMI (Electromagnetic Interference) | Cables near power lines or devices | CRC errors, unstable link |
| Bend radius violation | Sharp bends damaging cable pairs | Link flaps or errors |
How to Check:
- Observe link lights (LEDs)
- Run
show interfacesto see errors increasing - Use tools like a cable tester or TDR (Time Domain Reflectometer) on some Cisco switches:
test cable-diagnostics tdr interface gigabitEthernet0/1
7️⃣ Commands Summary for Troubleshooting
| Command | Purpose |
|---|---|
show interfaces | Displays detailed interface statistics (speed, duplex, errors) |
show interfaces status | Shows summary of interface state, speed, and duplex |
show controllers ethernet-controller | Displays hardware-level statistics |
show running-config interface <int> | Check interface configuration |
test cable-diagnostics tdr interface <int> | Tests physical cable conditions |
✅ Key Points for CCNA Exam
- Collisions = Data overlap (common in half-duplex)
- Errors (CRC, input, frame) = Damaged or corrupted frames
- Duplex mismatch = One side full-duplex, the other half → causes collisions and errors
- Speed mismatch = Different speeds cause link or communication failure
- Cabling issues = Poor or damaged cable, wrong type, or interference
Remember:
✅ Modern Ethernet networks should have zero collisions.
✅ Always check speed, duplex, and cable quality when troubleshooting link problems.
