JN0-106 Networking Fundamentals Practice Question
You are troubleshooting a network connectivity issue at a medium-sized company that uses Juniper EX4300 switches. Users in VLAN 100 (10.10.100.0/24) report that they cannot access a server in VLAN 200 (10.10.200.0/24). The switch has IRB interfaces configured for both VLANs and is acting as the default gateway. The IRB interfaces are up and have correct IP addresses. The server and users have correct IP configurations. 'show arp' on the switch shows incomplete entries for some hosts in VLAN 100. 'show ethernet-switching table' shows MAC addresses for users are present on the correct access ports. However, pings from the switch to a user in VLAN 100 fail. What is the most likely issue?
⚠ Common exam trap
Many candidates confuse a Layer 2 issue (like STP blocking or missing MAC entries) with a Layer 3 issue (ARP resolution failure), overlooking that 'incomplete' ARP entries directly point to a host not responding to ARP requests.
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 switch cannot resolve ARP requests because the user's host is not responding.
The 'incomplete' entries in the 'show arp' output indicate that the switch sent ARP requests for hosts in VLAN 100 but received no replies. Since the switch is the default gateway, it must resolve the Layer 2 MAC address of each host to forward traffic. If a user's host is not responding to ARP (e.g., due to a firewall, misconfiguration, or the host being offline), the switch cannot complete the ARP cache entry, causing pings from the switch to fail and potentially disrupting inter-VLAN routing.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Spanning Tree Protocol is blocking the user ports.
Why it's wrong here
Spanning Tree Protocol (STP) blocking the user ports would place those interfaces in a BLK/alternate state, preventing the switch from sending or receiving frames on them. In that state, the switch would not transmit ARP requests on those ports, so it would not record an incomplete ARP entry for the user host; instead, the ARP entry would be absent entirely. Since the symptom is an ARP entry stuck in 'incomplete', STP is not the cause—that state indicates the switch actively sent ARP requests but received no replies.
- ✗
A static MAC address entry for the server is missing.
Why it's wrong here
The issue is with the switch trying to resolve the user's IP address to a MAC address via ARP, not with forwarding to the server. ARP entries are populated dynamically from received packets, and a static MAC address entry for the server would only matter if the switch needed to reach the server's MAC directly without an ARP exchange. If the server's MAC were missing, traffic to the server would fail with 'no route to host' or a different symptom, not an incomplete ARP entry for a user host in VLAN 100.
- ✓
The switch cannot resolve ARP requests because the user's host is not responding.
Why this is correct
When a switch's ARP table shows an entry as 'incomplete', it means the switch has sent ARP requests for that IP address but has not received any ARP reply. This occurs when the target host is down, has the wrong IP address, or is configured with a firewall that silently drops ARP probes. The user's host is not responding to the ARP requests, so the switch cannot determine the MAC address needed to forward frames, breaking connectivity even though the host may be physically connected.
- ✗
The IRB interface for VLAN 100 is not configured with a proxy ARP.
Why it's wrong here
Proxy ARP is a feature where a Layer 3 device responds to ARP requests on behalf of another device, typically one on a different subnet. In this scenario, the switch's IRB interface for VLAN 100 and the user host are on the same subnet, so the user host should respond to ARP requests directly—proxy ARP is unnecessary for local subnet communication. Disabling or omitting proxy ARP does not prevent the switch from resolving ARP for a directly connected host; it only affects resolution across subnets when routing is involved.
Visual reference
Quick reference
Access Control Model Comparison
| Model | Acronym | Who Controls Access? | Best For |
|---|---|---|---|
| Discretionary Access Control | DAC | Resource owner | Small teams, file shares |
| Mandatory Access Control | MAC | System / security labels | Classified govt / military |
| Role-Based Access Control | RBAC | Administrator (via roles) | Enterprise environments |
| Attribute-Based Access Control | ABAC | Policy engine (user + resource attributes) | Fine-grained, dynamic policies |
| Rule-Based Access Control | RuBAC | System rules / ACLs | Firewall rules, network ACLs |
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This JN0-106 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 JN0-106 exam.