JNCIS-ENT Practice Question: Troubleshooting a connectivity issue between an EX4300 switch access port and an end-host
You are troubleshooting a connectivity issue between an EX4300 switch access port and an end-host. The port is configured as an access port for VLAN 100, but the host cannot communicate. You run 'show ethernet-switching interface ge-0/0/1' and notice the port status is down/down. Which underlying reason most likely explains this state?
⚠ Common exam trap
The trap here is conflating logical misconfiguration (VLAN not created, native VLAN mismatch) with a physical-layer failure — candidates often assume a config error causes down/down, but down/down is almost always a Layer 1 problem.
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 physical cable is disconnected or the transceiver is faulty.
A port status of down/down in Junos means the physical layer is not operational — the link itself has no signal. This is distinct from a logical or configuration problem, which would typically show up/down or down/up. A disconnected cable, faulty transceiver, or bad port are the classic causes of down/down. VLAN or BPDU issues would not produce a down/down state because they operate above the physical layer.
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 native VLAN ID does not match the upstream trunk switch.
Why it's wrong here
Native VLAN settings apply to trunk ports, and a mismatch causes traffic to land in the wrong VLAN or spanning tree inconsistencies, while the physical links stay up. This port is an access port, and down/down means there is no physical link at all, so a native VLAN mismatch cannot explain it. Layer 1 has to be fixed before any VLAN setting matters.
- ✗
BPDU protection has disabled the port due to a loop.
Why it's wrong here
BPDU protection on Junos does disable a port when a BPDU is received, but the switch reports it as blocked or disabled by BPDU protection in spanning tree and Ethernet switching output, a distinct state with its own reason. down/down instead means no link signal, typically a disconnected or faulty cable, a bad transceiver, or a host interface that is off. Check 'show interfaces ge-0/0/1 extensive' first.
- ✗
The VLAN 100 has not been created under the [edit vlans] hierarchy.
Why it's wrong here
If VLAN 100 were missing, the configuration referencing it would fail to commit or the port would not belong to a working VLAN, but the physical link would still come up, showing up/up. down/down is a Layer 1 condition, no link signal, so a VLAN configuration error cannot explain it. Fix the physical link first, then confirm VLAN membership with 'show vlans' or 'show ethernet-switching interface'.
- ✓
The physical cable is disconnected or the transceiver is faulty.
Why this is correct
down/down means the physical link is not established: the switch sees no signal from the host. The usual causes are a disconnected or damaged cable, a faulty or incompatible SFP transceiver, or a host network interface that is disabled or broken. Check the port and host LEDs, try a known-good cable or optic, and use 'show interfaces ge-0/0/1 extensive' or the optics diagnostics command to inspect link details.
Visual reference
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Written and reviewed by Johnson Ajibi, MSc IT Security
Security Systems Architect · founder of Courseiva
Last reviewed September 2026 · checked against the official Juniper Networks exam blueprint
This JNCIS-ENT practice question is part of Courseiva's free Juniper Networks certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the JNCIS-ENT exam.