show interface

5.4 Troubleshooting Tools

Basic Network Device Commands

📘CompTIA Network+ (N10-009)

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1. What is show interface?

The show interface command is a basic network device command used on network devices like switches and routers.

• It displays detailed information about all network interfaces (ports) on the device.
• An interface is a physical or logical connection point on the device, such as FastEthernet 0/1 or GigabitEthernet 0/1.

This command is extremely useful for troubleshooting network issues, because it shows the status, performance, and errors of the interfaces.

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2. Why it is important for the exam

For the CompTIA Network+ exam, you should be able to:

• Identify interface status and problems.
• Understand what different outputs mean (like up, down, or errors).
• Use it to diagnose connectivity issues.

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3. What show interface shows

When you run the show interface command, you get a lot of information. Here’s a breakdown of the key items:

A. Interface Status

• Administrative status: up or down – whether the interface is enabled in configuration.
• Operational status: up or down – whether the interface is actually working (connected and sending/receiving data).

Example:

FastEthernet0/1 is up, line protocol is up

• is up → interface enabled (administrative)
• line protocol is up → interface working (operational)

If either is down, the interface is not fully operational.

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B. IP Address

• Shows the IP address assigned to the interface (if any).
• Useful for checking if a device has the correct IP to communicate on the network.

Example:

Internet address is 192.168.1.1/24

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C. Traffic Statistics

• Shows how much data is being sent and received.
• Includes:
  • Packets sent/received
  • Bytes sent/received
  • Errors, such as:
    • Collisions – multiple devices trying to send data at the same time (mostly on old Ethernet)
    • Input errors – problems receiving data (bad cables, misconfigurations)
    • CRC errors – corrupted packets

Example:

5 packets input, 0 CRC, 0 collisions

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D. Speed and Duplex

• Speed: 10Mbps, 100Mbps, 1Gbps, etc.
• Duplex: Half or Full
  • Full-duplex → can send and receive at the same time.
  • Half-duplex → can only send or receive at a time.

Mismatched speed/duplex between devices can cause slow performance or errors.

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E. Interface Type

• Shows whether it’s FastEthernet, GigabitEthernet, Serial, or Loopback.
• Important for identifying what type of network connection you are troubleshooting.

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F. Layer 1 and Layer 2 Status

• Layer 1 (Physical) → cable or port physically working (up or down).
• Layer 2 (Data link) → protocols like Ethernet or PPP working (up or down).

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4. Common Uses in IT Environments

Network engineers use show interface to:

1. Check connectivity problems
   • If a user can’t connect, check if the interface is up/up.
2. Verify configuration
   • Confirm correct IP address, speed, and duplex.
3. Monitor traffic and errors
   • Detect faulty cables, collisions, or packet loss.
4. Troubleshoot slow network
   • Look for high errors or mismatched duplex.

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5. Exam Tips

For the CompTIA Network+ exam:

• Know the difference between administrative status and operational status.
• Be able to identify common errors shown in show interface.
• Understand the meaning of speed and duplex settings.
• Know that show interface is for troubleshooting and monitoring interfaces.

Remember: This command is often paired with others like ping, traceroute, and show ip route to troubleshoot network issues.

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6. Example Output and Explanation

FastEthernet0/1 is up, line protocol is down
  Hardware is FastEthernet, address is 000a.f7d2.1b2c
  Internet address is 192.168.1.10/24
  MTU 1500 bytes, BW 100000 Kbit/sec, DLY 100 usec
  10 packets input, 0 errors, 0 drops, 0 overruns
  12 packets output, 0 errors, 0 collisions
  Full-duplex, 100Mb/s, media type is RJ45

Explanation:

• up/down → Admin is up but interface is not operational → check cable or remote device.
• IP address is assigned.
• No errors → good physical layer.
• Full-duplex 100Mb/s → correct network settings.

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✅ Key Points to Remember for the Exam

1. show interface → checks status, traffic, errors, speed/duplex, and IP.
2. up/up → healthy interface.
3. Errors → may indicate cable or configuration problems.
4. Essential for troubleshooting network connectivity.