Interface Issues: Errors, Discards, Duplex, Speed, Port Status, and PoE
Key Takeaways
- Interface counters help separate physical errors, congestion drops, administrative shutdowns, and policy blocks.
- Errors such as CRC, runts, giants, and late collisions often point to physical media or duplex problems.
- Discards can occur when a device intentionally drops frames because queues, buffers, QoS, or policy cannot forward them.
- Duplex and speed mismatches can cause poor throughput even when the link light is on.
- PoE troubleshooting includes power budget, device class, cable quality, port configuration, and standards compatibility.
Interface Issues and Counters
An interface can be up and still unhealthy. Network+ troubleshooting expects you to read clues from switch, router, firewall, access point, and endpoint interfaces. The most useful questions are: is the port administratively enabled, is link present, did speed and duplex negotiate correctly, are errors increasing, are packets being discarded, and is power being delivered when required?
Port Status
| Status clue | Meaning | Next check |
|---|---|---|
| Administratively down | Port was disabled by configuration | Verify change records, enable if approved |
| Down/down | No Layer 1 link | Check cable, endpoint, transceiver, and port |
| Up/down or err-disabled | Link or protocol problem depending on platform | Check errors, security violation, STP, UDLD, or keepalive behavior |
| Up/up but no traffic | Link is present but traffic path may fail | Check VLAN, ACL, IP settings, ARP, and counters |
| Flapping | Link repeatedly rises and falls | Check cable, optic, power, endpoint NIC, and logs |
Port security, loop protection, BPDU guard, storm control, or error-disable features can intentionally shut a port after a violation. That is different from a broken cable, even though users may report both as "the network is down."
Errors and What They Suggest
| Counter or symptom | Common cause |
|---|---|
| CRC errors | Bad cable, interference, dirty fiber, duplex mismatch |
| Runts | Frames smaller than expected, often collision or NIC/cabling issue |
| Giants | Frames larger than allowed MTU, jumbo frame mismatch |
| Late collisions | Duplex mismatch or excessive segment length on half-duplex Ethernet |
| Input errors | General receive problems, often physical or framing related |
| Output errors | Transmit problems, congestion, or interface hardware issue |
Errors that increase steadily under load are more useful than old counters left from a previous incident. Clear counters only when allowed by local practice and then reproduce or observe the issue.
Discards
Discards are not always physical errors. A device can drop otherwise valid traffic because it has no buffer space, the egress queue is congested, QoS policy drops lower-priority traffic, or a security policy denies forwarding.
| Discard type | Troubleshooting clue |
|---|---|
| Input discards | Device cannot process received frames fast enough or policy drops them |
| Output discards | Egress queue congestion or shaping/policing behavior |
| QoS drops | Lower-priority traffic dropped during congestion |
| ACL or firewall drops | Policy intentionally denies traffic |
When users report slow performance and output discards climb on an uplink, congestion is more likely than a bad endpoint IP configuration.
Duplex and Speed Mismatch
Autonegotiation normally sets speed and duplex. Problems appear when one side is hard-coded and the other negotiates, or when cabling cannot support the desired rate. A classic duplex mismatch has one side full-duplex and the other half-duplex. Symptoms include poor throughput, collisions on the half-duplex side, CRC errors, and complaints that small pings work but file transfers fail.
| Symptom | Likely issue |
|---|---|
| Link negotiates 100 Mbps instead of 1 Gbps | Pair fault, cable rating, port limit, or forced setting |
| High collisions on one side | Half-duplex behavior or duplex mismatch |
| Good ping but bad bulk transfer | Duplex mismatch, errors, or congestion |
| Speed fixed on one side only | Autonegotiation mismatch risk |
PoE Troubleshooting
Power over Ethernet supplies power to devices such as phones, cameras, access points, badge readers, and IoT devices. A device can fail because data works but power does not, or because power works until the device draws more under load.
| PoE clue | Check |
|---|---|
| Device never powers on | PoE enabled, power standard, port budget, cable pairs |
| Device boots then reboots | Insufficient power class, budget exhaustion, cable loss |
| Some APs fail after adding new devices | Switch power budget exceeded |
| Injector works but switch port does not | Switch PoE capability or configuration |
| Data link up but no power | Non-PoE port, disabled PoE, or incompatible powered device |
Exam Focus
For N10-009, do not stop at "the port is up." Use counters and status. Errors point toward media, duplex, or framing. Discards point toward congestion, queues, QoS, or policy. PoE issues require checking both the electrical power path and the Ethernet data path.
A switch port is up, but file transfers are extremely slow. One side is forced to full duplex and the other side autonegotiated half duplex. What is the likely problem?
Output discards on a switch uplink rise during busy periods, but physical error counters stay at zero. What is the most likely cause?
Match each interface clue to the most likely troubleshooting area.
Match each item on the left with the correct item on the right