network-interface-health

Topics in the source

• Network Interface Health
• When to Use
• How It Works
• Counter Reference
• Diagnosis Flow
• CRCs Or Input Errors
• Drops
• Duplex And Speed

Use this skill when a network symptom might be caused by a physical link, switch port, cable, transceiver, duplex setting, or congested interface. A host or VLAN has packet loss, latency spikes, or intermittent reachability. A switch or router interface shows CRCs, runts, giants, drops, resets, or flaps. You need to compare both ends of a link before replacing hardware. A change window needs before/after interface counter evidence. Monitoring reports rising ifInErrors, ifOutErrors, or ifOutDiscards. Interface counters are evidence, but the trend matters more than the absolute number. Capture a baseline, wait a measurement interval, capture again, then compare increments. On Linux hosts: | Counter | Meaning | Common cause | | --- | --- | --- | | CRC | Received frame checksum failed | Bad cable, dirty fiber, bad optic, duplex mismatch | | input errors | Aggregate receive-side errors | Check sub-counters before concluding | | runts | Frames below minimum Ethernet size | Duplex mismatch, collision domain, faulty NIC | | giants | Frames larger than expected MTU | MTU mismatch or jumbo-frame boundary | | input drops | Device could not accept inbound packets | Burst, oversubscription, CPU

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