show interfaces – Detailed Interface Statistics & Error Analysis

Purpose of show interfaces Command

Definition: show interfaces is a Cisco IOS command used to display detailed status, real-time statistics, and error counters for each interface.
Importance: It is crucial for:

  • Monitoring interface health and utilization.
  • Diagnosing connectivity and physical issues.
  • Identifying sources of packet loss, errors, or hardware faults.

Key Sections in the Output

  1. Interface Status: Whether the interface and protocol are up/down.
    GigabitEthernet0/1 is up, line protocol is up
    Both hardware (L1) and protocol (L2) are working.
  2. Hardware and MAC Address: Interface hardware type and MAC address.
    Example: Hardware is Gigabit Ethernet, address is 001a.2b3c.4d5e
  3. IP Address Configuration: If Layer 3, shows IP address and subnet mask.
  4. Traffic Statistics: Packets/bytes received and sent, plus rates.
  5. Error Counters:
    • Input errors: CRC, runts, giants, frame, overruns.
    • Output errors: Collisions, late collisions, deferred, babble, queue drops.
  6. Interface Utilization and Load: Bandwidth, packets/sec, 1- and 5-min load averages.
  7. Collisions and Duplex Mismatches: Collisions = half-duplex or misconfig.
  8. Reliability and MTU: Health indicator (255/255), MTU size.
  9. Queues and Drops: Packets dropped due to full input/output queues.
  10. Link Negotiation and Speed: Current speed/duplex (manual or negotiated).
  11. Physical Layer Issues: Up/down status and hardware errors indicate cabling or module faults.

Common Problems and Indicators

Symptom Key Output Section Probable Cause Solution
Up/Down or Down/Down Status line Cabling, device, admin shutdown Check cables, port config, remote end
High CRC/frame errors Input errors Bad cables, EMI/interference Replace cables, check for interference
Collisions/late collisions Output errors Duplex mismatch, shared media Set both sides to full-duplex
Runts/Giants Input errors MTU mismatch, faulty devices Check MTU, replace hardware
Input/output drops Queues and drops Congestion, undersized buffers Optimize traffic, upgrade interface
Low reliability Reliability Ongoing errors/hardware issues Investigate errors, replace hardware

Related Commands

CommandDescription
show interfaces status Concise status (up/down, speed, duplex, VLAN for switches)
show interfaces counters errors Focused error statistics
show interfaces [type number] Details for a specific interface

Sample Output and Interpretation

Switch# show interfaces GigabitEthernet0/1

GigabitEthernet0/1 is up, line protocol is up
  Hardware is Gigabit Ethernet, address is 001a.2b3c.4d5e
  Internet address is 192.168.10.2/24
  MTU 1500 bytes, BW 1000000 Kbit, DLY 10 usec,
  Full-duplex, 1000Mb/s, auto-negotiation
  123456789 packets input, 0 input errors, 0 CRC, 0 frame, 0 overruns
  234567890 packets output, 0 output errors, 0 collisions, 0 late collisions
  0 input queue drops, 0 output queue drops
  Reliability 255/255, txload 1/255, rxload 1/255
Interpretation: Interface is healthy—no errors, full duplex, no congestion or drops.

Common Interface Error Counters: Quick Reference

Error Counter Meaning Probable Cause Action/Resolution
Input errors Total errors in received packets (sum of runts, giants, CRC, etc.) Faulty cables, interference, hardware issue Replace cables, check for interference
CRC errors Packets failed CRC (Cyclic Redundancy Check); data corruption detected Damaged cables, EMI (electromagnetic interference) Replace/relocate cables
Frame errors Frames with incorrect size or alignment Collisions, cabling, hardware Inspect cables, fix duplex/speed
Runts Packets smaller than min Ethernet size (<64 bytes) Collisions, faulty NIC, cabling Fix NIC, cables, check duplex
Giants Packets larger than max Ethernet size (>1518 bytes) MTU mismatch, faulty device Check MTU settings, replace hardware
Overruns Interface ran out of buffer before processing packet High traffic, hardware bottleneck Increase buffer, optimize traffic
Input queue drops Packets dropped due to full input queue Congestion, insufficient CPU Investigate traffic, upgrade hardware
Output errors Errors on transmitted packets (sum of underruns, collisions, etc.) Duplex mismatch, overloaded interface Fix duplex, monitor load
Collisions Ethernet frames colliding on wire Half-duplex, shared media, duplex mismatch Set both ends to full duplex
Late collisions Collisions detected after 64 bytes of a frame Duplex mismatch, long cable Fix duplex, shorten cable
Deferred Frames delayed before sending due to busy network High utilization Monitor/manage traffic
Babble Excessively long frames sent by interface Malfunctioning hardware Replace hardware
Output queue drops Packets dropped due to full output queue Interface congestion, high traffic Upgrade interface or reduce traffic
Reliability Running measure of success (255/255=perfect) Ongoing errors, hardware failure Investigate persistent issues

Quick Troubleshooting Steps

  1. Check interface status: show interfaces [type number] (e.g., show interfaces GigabitEthernet0/1)
  2. Identify error counters in output.
  3. Correlate errors with symptoms:
    • Slow network: High drops/overruns
    • Disconnects: CRC/frame errors
    • No connectivity: down/down or administratively down
  4. Take corrective action: Replace cables/hardware, fix speed/duplex, reduce congestion.
  5. Clear counters after fixes, and monitor for recurrence.

Exam Tips and Key Points

  • Match error counters to their root cause and best resolution.
  • Duplex mismatches cause high collision/late collision counts.
  • Use queue drops/load to spot congestion.
  • Check both physical (Layer 1) and protocol (Layer 2/3) status.
  • Compare show interfaces before and after changes to verify improvement.

Show Interfaces Quiz

1. What does the show interfaces command primarily display?

Correct answer is B. The command shows detailed info about each interface, including errors and traffic statistics.

2. What does it mean if an interface status shows "GigabitEthernet0/1 is up, line protocol is up"?

Correct answer is C. Both Layer 1 (hardware) and Layer 2/3 (protocol) are operational.

3. Which error counter typically indicates data corruption caused by cabling issues?

Correct answer is A. CRC errors usually signify bad cabling or electromagnetic interference.

4. What is a common cause of high collisions and late collisions on an interface?

Correct answer is D. Duplex mismatch between devices causes collisions and late collisions.

5. What do runts and giants indicate on an interface?

Correct answer is B. Runts are too small; giants are too large packets, often indicating cabling or MTU issues.

6. What does "0 input queue drops" signify?

Correct answer is A. It means the input queue is not dropping packets, so no congestion or overload.

7. Which command provides concise status including VLAN, speed, and duplex on a Cisco switch?

Correct answer is C. The "show interfaces status" command summarizes key Layer 2 info on switch ports.

8. What does a reliability value of 255/255 indicate on an interface?

Correct answer is D. Reliability 255/255 means no errors and perfect health.

9. What should you do if you see high input errors and CRC errors on an interface?

Correct answer is A. CRC and input errors often stem from cabling or EMI issues, so physical checks are needed.

10. Which of the following is a symptom of a duplex mismatch?

Correct answer is B. Duplex mismatches cause many collisions and late collisions due to transmission issues.

← Back to Home