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Network Interface Health

Use this skill when a network symptom might be caused by a physical link, switch port, cable, transceiver, duplex setting, or congested interface.

When to Use

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.

How It Works

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.

show interfaces <interface>
show interfaces <interface> status
show logging | include <interface>|changed state|line protocol

On Linux hosts:

ip -s link show <interface>
ethtool <interface>
ethtool -S <interface>

Counter Reference

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 path, queue pressure
output drops	Egress queue discarded packets	Congestion, QoS policy, undersized uplink
resets	Interface hardware reset	Flapping, keepalive, driver, optic, power
collisions	Ethernet collision counter	Half duplex or negotiation mismatch

Diagnosis Flow

CRCs Or Input Errors

Confirm counters are incrementing, not just historical.
Check both ends of the link. Receive-side errors usually point to the signal arriving on that side, not necessarily the port reporting the error.
Replace patch cable or clean/replace fiber and optics.
Confirm speed/duplex settings match on both sides.
Check logs for flap events around the same timestamp.

Drops

Separate input drops from output drops.
Compare interface rate against capacity.
Check QoS policy, queue counters, and whether the link is an oversubscribed uplink.
Treat queue tuning as secondary. First prove whether the link is congested.

Duplex And Speed

Prefer auto-negotiation on modern Ethernet links when both sides support it. If one side must be fixed, configure both sides explicitly and document why. Never mix fixed speed/duplex on one side with auto on the other.

show interfaces <interface> | include duplex|speed

Safe Parser Example

Slice each interface block from one header to the next. Do not use an arbitrary character window; large interface blocks can cause counters to be missed or assigned to the wrong port.

import re
from typing import Any

HEADER_RE = re.compile(
    r"^(?P<name>\S+) is (?P<status>(?:administratively )?down|up), "
    r"line protocol is (?P<protocol>up|down)",
    re.I | re.M,
)
ERROR_RE = re.compile(r"(?P<input>\d+) input errors, (?P<crc>\d+) CRC", re.I)
DROP_RE = re.compile(r"(?P<output>\d+) output errors", re.I)
DUPLEX_RE = re.compile(r"(?P<duplex>Full|Half|Auto)-duplex,\s+(?P<speed>[^,]+)", re.I)

def parse_show_interfaces(raw: str) -> list[dict[str, Any]]:
    headers = list(HEADER_RE.finditer(raw))
    interfaces = []
    for index, header in enumerate(headers):
        end = headers[index + 1].start() if index + 1 < len(headers) else len(raw)
        block = raw[header.start():end]
        errors = ERROR_RE.search(block)
        drops = DROP_RE.search(block)
        duplex = DUPLEX_RE.search(block)
        interfaces.append({
            "name": header.group("name"),
            "status": header.group("status"),
            "protocol": header.group("protocol"),
            "duplex": duplex.group("duplex") if duplex else "unknown",
            "speed": duplex.group("speed").strip() if duplex else "unknown",
            "input_errors": int(errors.group("input")) if errors else 0,
            "crc_errors": int(errors.group("crc")) if errors else 0,
            "output_errors": int(drops.group("output")) if drops else 0,
        })
    return interfaces

Examples

CRCs On One Switch Port

Capture counters on the local port.
Capture counters on the connected remote port.
Replace the cable or optic before changing routing or firewall rules.
Clear counters only after recording the baseline.
Recheck after a fixed interval.

Internet Slow But LAN Is Fine

Check WAN interface drops/errors.
Check LAN uplink utilization and output drops.
Check gateway CPU if the WAN link is clean but throughput is still low.
Compare wired and wireless tests before blaming upstream service.

Anti-Patterns

Clearing counters before saving a baseline.
Looking at only one side of a link.
Assuming all historical CRCs are active problems without a time window.
Mixing auto-negotiation on one side with fixed speed/duplex on the other.
Treating output drops as a cable problem before checking congestion.

💬 Network Interface Health

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