CompTIA Network+ guideNetwork troubleshooting
CompTIA Network+ Cable and Interface Issues: CRC Errors, Port Status, and PoE
How to read N10-009 objective 5.2 problems from the physical layer up: media and connectors, rising interface counters, the three port states, PoE budget math, and transceiver mismatches.
Short answer
Objective 5.2 of CompTIA Network+ N10-009 covers cable issues (wrong cable type, signal degradation, bad terminations, TX and RX transposed), interface issues (rising CRC, runt, giant, and drop counters, and ports that are error-disabled, administratively down, or suspended), and hardware issues (PoE budget or standard, transceiver mismatch, and signal strength). Read counters as a rate, not a total, and tell a port the switch shut from one an administrator shut. A switch with a 370 W PoE budget can power 14 devices drawing 25 W each, not 16.
What N10-009 objective 5.2 covers, and why media from 1.5 belongs here
Network Troubleshooting is the largest domain on CompTIA Network+ N10-009 (V9), at 24% of the exam. Objective 5.2 within it is a "given a scenario" objective about cabling and physical interfaces. It groups problems into three kinds: cable issues, interface issues, and hardware issues such as PoE and transceivers.
Objective 1.5, which compares transmission media and transceivers, sits in Networking Concepts. It is easier to learn next to its failure modes, so this page covers both. In the troubleshooting methodology, these checks are where a bottom-to-top pass through the OSI model starts.
Transmission media and connectors in N10-009 (objective 1.5)
This table leaves out distance and speed figures on purpose. Take those from the datasheet of the cable or optic you are using.
| Media | What it is | Where you usually find it |
|---|---|---|
| Single-mode fiber | A narrow core that carries one path of light | Longer runs, such as between buildings |
| Multimode fiber | A wider core that carries many paths of light | Shorter runs inside a building or data center |
| Direct attach copper (DAC), twinaxial | Copper cable with a transceiver fixed on each end | Short links between switches and servers in the same or next rack |
| Coaxial | A center conductor inside a shield | Cable broadband and some video links |
| Plenum vs. non-plenum | A jacket rated, or not rated, for air-handling spaces | Plenum cable where building codes require it |
The connectors in the objectives split by media. SC, LC, ST, and MPO are fiber connectors. SC is a square push-pull connector, LC is the small connector common on SFP optics, ST is a round twist-lock connector, and MPO carries many fibers in one connector. RJ11 and RJ45 are modular connectors for telephone and twisted-pair Ethernet. F-type and BNC are coaxial connectors. Transceivers come in SFP and QSFP form factors and speak Ethernet or Fibre Channel.
Cable problems: wrong cable, degraded signal, bad termination, TX/RX
- Incorrect cable. The objectives name single-mode vs. multimode, Category 5/6/7/8, and shielded vs. unshielded twisted pair. A link that comes up at a lower speed than expected, or not at all, often has the wrong cable somewhere in the path.
- Signal degradation. Crosstalk is signal leaking between pairs, often where pairs were untwisted too far at a termination. Interference comes from outside sources such as motors, lighting, and power cables. Attenuation is signal lost over distance, as on a run longer than its rating.
- Improper termination. Pairs in the wrong order, loose crimps, or too much untwisting. A cable tester shows opens, shorts, and miswires.
- TX/RX transposed. On fiber, one end's transmit strand must reach the other end's receive strand. If the pair is crossed the wrong way, the link stays down until you swap the two strands at one end. Many copper ports detect and correct a crossed pair on their own, so in practice this is mostly a fiber symptom.
The 5.5 hardware tools match these problems. A toner traces a cable to its jack, a cable tester checks the wiring, a tap copies traffic for analysis, and a visual fault locator shines visible light into fiber so you can see a break or a tight bend. The objectives' proposed lab list also names an optical power meter.
Interface counters: what rising CRC, runts, giants, and drops mean
The objectives say "increasing interface counters," and the word that matters is increasing. A counter is a running total since the last reset. An old total tells you something went wrong at some point. A total that keeps climbing tells you it is going wrong now.
| Counter | What it counts | Likely causes | First check |
|---|---|---|---|
| CRC | Frames whose checksum fails on arrival, meaning the data was damaged on the way | A bad cable or connector, interference, a failing transceiver, or a duplex mismatch | Swap the patch cable, then compare speed and duplex on both ends |
| Runts | Frames shorter than the Ethernet minimum | Collisions caused by a duplex mismatch, or a faulty NIC | Duplex settings on both ends |
| Giants | Frames larger than the port accepts | An MTU mismatch, such as jumbo frames enabled on one end only | MTU on both ends of the link |
| Drops | Frames discarded, often because a queue was full | Congestion, a fast port feeding a slower one, or a QoS policy | Utilization and queues, covered in the performance troubleshooting guide |
Worked reading. At 9:00 a.m., show interface on a switch port reports 1,204 CRC errors. At 9:10 a.m. it reports 1,876. That is 672 new errors in 10 minutes, or about 67 a minute, so the problem is active. The technician swaps the patch cable at 9:15 and notes the counter. At 9:25 it has not moved. Had the 9:10 reading still been 1,204, the errors would have been history, and replacing hardware would have fixed nothing.
Port status on Network+: error-disabled, administratively down, suspended
The three states look alike in a status list but have different owners.
| State | Who put the port there | What to do |
|---|---|---|
| Error-disabled | The switch, after it detected a problem | Find and fix the cause, then re-enable the port |
| Administratively down | An administrator, with a shutdown command | Check the change records. Re-enable it only if it should be in service |
| Suspended | On many switches, a link-aggregation member the switch has pulled out of its bundle | Compare the aggregation settings on both ends |
Cisco's IOS documentation, updated October 7, 2026, lists causes of the err-disabled state that include port security violations, duplex mismatch, BPDU guard, link flapping, and an incorrect SFP or cable. It says to fix the cause first, or the port will error-disable again, then recover with shutdown followed by no shutdown. Automatic recovery is off by default. Those commands are Cisco's, and other vendors name things differently, but the order holds: cause first, then re-enable.
The free mixed set of 20 Network+ questions includes an item where a desk port shows err-disabled after a user changes what is plugged into it. You can test yourself on the err-disabled desk-port scenario, with an explanation for every option.
PoE problems: power budget exceeded, worked, and incorrect standard
A PoE switch has a total power budget shared by all its powered ports. The numbers below are example datasheet values, not standards.
A switch has a 370 W PoE budget, and each access point requires a 25 W allocation from that switch budget, including the allowance for cable loss.
- 370 ÷ 25 = 14.8, so the switch can power 14 access points at full draw.
- 14 × 25 = 350 W, which leaves 20 W of the budget.
- Sixteen access points need 16 × 25 = 400 W, which is 30 W over budget.
What happens to the ports beyond the budget depends on the switch's power policy. Plan with the maximum draw, because devices that draw less on a quiet day can draw more under load. The show power command listed in objective 5.5 is the generic form of the command that reports the budget and per-port draw.
An incorrect standard is a different problem. A device that needs more power than the port's PoE standard can supply may not power on, or may come up with features turned off, depending on the device. Compare the device's requirement with the port's rating in both datasheets.
Transceivers: mismatch and signal strength
The optics at both ends of a link have to agree with each other and with the fiber between them. A single-mode optic on one end and a multimode optic on the other, or an optic that does not match the fiber type, gives no link or a link full of errors. Some switches also refuse transceivers they do not support.
Low signal strength usually comes from dirty or damaged connectors, tight bends, too many patch points, or a run longer than the optic is rated for. Clean and inspect the connectors before replacing anything.
A bottom-up pass through one Network+ link failure
A fiber uplink between a building's MDF and an IDF on another floor (a fictional site) stays down after a switch replacement.
- Port state. The new switch shows the port as down, not administratively down and not err-disabled.
- Transceivers. The replacement switch came with multimode optics, but the riser fiber is single-mode. The technician fits single-mode optics. The link still stays down.
- TX/RX. A visual fault locator shows light arriving at the far panel on the expected strands, which shows continuity on those strands but does not certify acceptable optical loss or performance. Swapping the two strands at one end brings the link up.
- Counters. CRC errors climb slowly. Cleaning both connectors stops them.
Every step used evidence from the layer below the next. For the commands behind each check, see the Network+ command-line tools guide. Speed, duplex, and MTU settings are configured as part of switching, covered in the switching guide.
Trust the source
Official sources
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