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6.2.2. Interface Counters and Port Status
Interface Counters to Watch:
| Counter | Indicates |
|---|---|
| CRC errors | Frame corruption—bad cable, interference |
| Runts | Frames too small—collision, bad NIC |
| Giants | Frames too large—MTU mismatch |
| Input/Output errors | General interface problems |
| Drops | Buffer overflow, congestion |
| Late collisions | Duplex mismatch—one side full duplex, the other half |
| Pause frames | IEEE 802.3x flow control—a congested receiver is asking the sender to pause |
Port Status:
- Error disabled: Switch disabled port due to violation—e.g., port security seeing more MAC addresses than allowed, or BPDU guard receiving a BPDU
- Administratively down: Manually disabled
- Suspended: VLAN mismatch or trunk negotiation issue
- Flapping: Port repeatedly goes up and down—loose connector, damaged cable, or failing NIC/SFP
Speed/duplex mismatch: If one end is hard-set and the other auto-negotiates, the auto side can fall back to half duplex. The half-duplex side then logs late collisions, the full-duplex side logs CRC errors and runts, and throughput on that link collapses. Configure both ends the same (both auto is best). A gigabit port that negotiates only 100 Mbps often has a damaged pair or older cable: 1000BASE-T needs all four pairs, 100BASE-TX uses only two.
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Written byAlvin Varughese
Founder•20 professional certifications