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CCNA 200-301 v1.1 reference — Network Fundamentals

Interface status and error troubleshooting

Read interface status codes and counters, and isolate collisions, CRC errors, duplex and speed mismatches, and cabling faults.

Objective 1.4v1.1 exam target
Quick ReferenceHandbook
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Interface status combinations

Line protocolStatusMeaningFirst suspects
up / upWorkingL1 and L2 operationalLook elsewhere — issue is higher in the stack
up / downL2 problemPhysical link OK, protocol failingEncapsulation mismatch, keepalive issues
down / downL1 problemNo carrier detectedCable, optic, far-end shutdown, bad port
administratively down / downShutdownshutdown configuredSomeone disabled it — no shutdown
down / down (err-disabled)Disabled by featurePort security, BPDU guard, link-flap, etc.show interface status err-disabled for reason

Error counters and what they indicate

CounterMeaningLikely cause
CRC / FCS errorsFrame checksum failed on receiptPhysical-layer suspect — compare counter deltas over time with cable/optic condition
RuntsFrames shorter than 64 bytesCollisions (half-duplex), duplex mismatch, faulty NIC
GiantsFrames over the interface MTUMTU mismatch, baby giants/jumbo misconfig
CollisionsContention on shared mediaHalf-duplex link, duplex mismatch
Late collisionsCollision detected after 512 bitsDuplex mismatch is the classic cause
Input/output errorsGeneral receive/transmit failuresBuffer overruns, physical faults
Interface health check

Baseline interface state and error counters — Privileged EXEC

Cisco IOS / IOS XE

Verify
show ip interface brief
show interfaces status
show interfaces gi0/1
show interfaces gi0/1 counters errors
  • show ip interface brief reveals status/protocol pairs at a glance.
  • show interfaces status exposes negotiated speed/duplex and err-disabled.
  • Error counters near zero and non-incrementing indicate a healthy physical link.
  • Counter snapshots are meaningless alone — clear and re-check to see increments.

Read-only verification commands.

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Handbook sections

Related resources

Practise this topicSource topic 9: Switch Interfaces & Error CountersPractise topic 9 deck

Sources

Targets CCNA 200-301 v1.1 objectives. Supplemental blocks are labeled beyond exam scope.

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Interface status and error troubleshooting | CCNA Study