RANGER CompTIA Network+ (N10-009) · chapter 25 of 29
Week 25: Blinking lights
In this chapter Objectives 5.2, 5.5
Objective 5.2
Interface counters and port status
Increasing interface counters
Rising counters on 'show interface' point to the cause:
- CRC: frames failed the checksum: bad cable, interference, duplex mismatch
- Runts: frames under 64 bytes, often collisions or duplex mismatch
- Giants: frames over 1518 bytes, often an MTU/jumbo frame mismatch
- Drops: frames discarded, usually full buffers from congestion
Exam tip Counters matter when they INCREASE. Clear them, wait, check again.
Port status
Not every down port is broken:
- Error disabled (err-disabled): the switch shut the port for a violation: port security, BPDU guard, flapping
- Administratively down: someone typed 'shutdown'; fix with 'no shutdown'
- Suspended: a port-channel (LACP) member whose settings don't match the bundle is held out of it
Gi1/0/7 is down, line protocol is down (err-disabled) Gi1/0/9 is administratively down, line protocol is down Po1: Gi1/0/49(P) Gi1/0/50(s) P = bundled, s = suspended
Exam tip Err-disabled after port security? Remove the cause, then shutdown / no shutdown (or use errdisable recovery).
Objective 5.2
Hardware issues: PoE and transceivers
Power over Ethernet
PoE standards: 802.3af (PoE) 15.4 W per port, 802.3at (PoE+) 30 W, 802.3bt (PoE++) 60 W (Type 3) and 90 W (Type 4, often sold as '100 W'), measured at the switch. Two classic problems: power budget exceeded (the switch's total PoE supply is used up) and incorrect standard (a PoE+ AP on an 802.3af-only port boots partially or not at all).
Exam tip AP shows a link but radios are off, or it keeps rebooting? Check PoE class vs. port and the switch budget.
Transceivers
Mismatch: both ends must use compatible optics: same wavelength and type (e.g., 10GBASE-SR multimode vs. 10GBASE-LR single-mode), matching speed and fiber. Signal strength: optics report transmit and receive power in dBm. Too low (dirty connector, long run, bad splice) or too high (short single-mode run with a long-reach optic) causes errors or no link.
Exam tip Clean fiber ends first; a dirty connector is the most common cause of low receive power.
Objective 5.5
Device commands: show interface and show power
show interface and show power
show interface gives status, speed/duplex, rates and error counters. show power (Cisco: show power inline) lists the PoE budget and per-port power.
WH-SW1# show power inline Available:370.0(w) Used:362.4(w) Remaining:7.6(w) Intf Oper Power Device Class Gi1/0/10 on 30.0 AP-WH-03 4 Gi1/0/11 on 15.4 CAM-WH-12 0 Gi1/0/12 off 0.0 n/a n/a
Exam tip Oper 'off' with a device attached and little power remaining = budget exceeded.
Check yourself 1A switch port shows thousands of increasing giants after a server was set to 9000-byte frames. What is the most likely cause?
- TX/RX transposed
- Duplex mismatch between server and switch
- MTU mismatch between server and switch
- Attenuation
Show the answer
Answer: C. MTU mismatch between server and switch
Giants are frames larger than the port expects. Jumbo frames on one side only produce them.
Practice questions
Answer in your head, then open the answer. Ranger's Packet Trail has 483 questions like these for Network+, with mock exams.
Question 1A user's wired connection drops only when the HVAC compressor kicks on. The technician finds that the user's patch cable is draped across the compressor housing. What is the MOST likely cause?
- Attenuation
- Electromagnetic interference
- Crosstalk
- Power budget exceeded
Show the answer
Answer: B. Electromagnetic interference
Drops that line up with a motor starting point to EMI from that equipment. Rerouting the cable or using shielded cable fixes it.
Question 2A technician punches down a new wall jack. The cable tester's wire map shows pins 1-2-3-6 pass but pin 7 is open at the jack end, and the conductor is visibly not seated in the IDC slot. What is the issue?
- Improper termination
- Attenuation
- Crosstalk
- Transceiver mismatch
Show the answer
Answer: A. Improper termination
A conductor not seated in the insulation-displacement contact is improper termination, and the wire map shows it as an open. Re-terminating the jack fixes it.
Question 3A new duplex LC fiber jumper between two switches shows no link. Both SFPs are the same type and show normal Tx power, but Rx power on both ends reads -40 dBm (no light). What should the technician try FIRST?
- Replace both SFPs with multimode optics
- Increase the switch's PoE budget
- Clean the connectors and reseat the jumper
- Swap the two strands at one end of the patch
Show the answer
Answer: D. Swap the two strands at one end of the patch
If TX is connected to TX, neither receiver gets light and the link stays down. Swapping the strands at one end puts each transmitter on the far receiver.
Question 4A user complains of slow file transfers. The technician runs a command on the switch port, shown in the exhibit. What is the MOST likely cause?
- An MTU mismatch is producing oversized jumbo frames
- A damaged cable or outside noise is corrupting frames
- A duplex mismatch with the PC is causing late collisions
- The port is congested and dropping output
Show the answer
Answer: B. A damaged cable or outside noise is corrupting frames
Input errors that are all CRC errors, with no runts, giants or collisions, mean frames arrive corrupted, usually from a bad cable, connector or EMI.
Question 5Which interface counter increases when a switch port receives Ethernet frames that are smaller than the 64-byte minimum?
- Runts
- Giants
- Output drops
- Throttles
Show the answer
Answer: A. Runts
Runts are frames shorter than 64 bytes, often caused by collisions or duplex problems. Giants are the opposite: frames above the maximum size.
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