JN0-106 User Interfaces Practice Question
Exhibit
Physical interface: ge-0/0/0, Enabled, Physical link is Up
Interface index: 145, SNMP ifIndex: 511
Link-level type: Ethernet, MTU: 1514, Speed: 1Gbps
Device flags : Present Running
Interface flags: SNMP-Traps Internal: 0x4000
Link type : Full-Duplex
Link flags : None
Input errors:
Errors: 0, Drops: 0, Framing errors: 0, Runts: 0, Giants: 0, Policed discards: 0, Resource errors: 0
Output errors:
Errors: 0, Drops: 0, Carrier transitions: 0, Collisions: 2546, Late collisions: 34, Deferred: 0, ...Refer to the exhibit. An engineer notices high error counts on the interface. Based on the output, what is the most likely cause?
⚠ Common exam trap
Many candidates assume high error counts always indicate a hardware fault (Option A), but in JNCIA-JUNOS, the specific error types (e.g., collisions, late collisions) point to a duplex mismatch, not a physical layer failure.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface is connected to a device configured for half-duplex.
The output shows high error counts on the interface, which is a classic symptom of a duplex mismatch. When one end of an Ethernet link operates in full-duplex and the other in half-duplex, collisions occur on the half-duplex side because it expects to wait for the carrier to be clear before transmitting, while the full-duplex side transmits at any time. This leads to frame check sequence (FCS) errors, alignment errors, and runts on the half-duplex interface. Option D is correct because the local interface is likely full-duplex (default on modern Juniper devices), and the connected device is configured for half-duplex, causing the mismatch.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
The interface has a hardware fault.
Why it's wrong here
A hardware fault is not the primary cause because the specific error type matters. Faulty hardware, cables, or optics typically generate CRC errors, FCS errors, alignment errors, or excessive symbol errors, not deliberate collision fragments. On a full-duplex interface, late collisions are a sign of protocol-level duplex mismatch rather than physical-layer hardware degradation, so this option cannot explain the observed counters.
- ✗
The interface is experiencing excessive broadcasts.
Why it's wrong here
Excessive broadcasts can overwhelm the control plane and raise CPU utilization, but a broadcast storm does not alter the electrical signaling or media access on a full-duplex point-to-point link. Full-duplex Ethernet uses dedicated transmit and receive pairs, so even a massive number of broadcast frames will not produce collision events or late-collision errors. Thus broadcast load cannot create the high collision counts visible in the exhibit.
- ✗
The interface is operating in half-duplex mode.
Why it's wrong here
The exhibit explicitly shows 'Link type: Full-Duplex', so the local interface is not operating in half-duplex mode. If the local interface were half-duplex, collisions could be expected because the transmit and receive channels share the same wire, but that contradicts the interface's negotiated mode. The error counters must therefore be explained by the behavior of the remote side, not by the local link mode stated in the output.
- ✓
The interface is connected to a device configured for half-duplex.
Why this is correct
When a local full-duplex interface connects to a remote half-duplex device, the remote cannot simultaneously transmit and receive, and it performs CSMA/CD to avoid collisions. If the remote begins transmitting while the full-duplex side is also transmitting, the half-duplex side detects a collision and may truncate or jam the frame; the full-duplex side then observes a late collision or undersized frame. Because the local link is full-duplex, collisions can only originate from a remote device operating half-duplex, making a duplex mismatch the correct explanation for the high error counts.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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