The answer is that the switch cannot resolve ARP requests because the user’s host is not responding. When a Juniper EX4300 switch acting as the default gateway for inter-VLAN routing shows incomplete ARP entries, it means the switch sent ARP requests to resolve the Layer 2 MAC address of a host but received no reply. Since the switch must map each IP address to a MAC address to forward traffic between VLANs via the IRB interfaces, an unresponsive host breaks the path—pings from the switch itself fail, and inter-VLAN routing fails for that host. On the JNCIA-Junos exam, this scenario tests your understanding of ARP’s role in Layer 3 forwarding and the difference between a Layer 2 switching table (which shows MACs on access ports) and a Layer 3 ARP table. A common trap is assuming the switch can route without a complete ARP entry, but remember: no ARP reply means no MAC, no MAC means no frame delivery. Memory tip: “ARP incomplete = host asleep or blocked.”
JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking fundamentals. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
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.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
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
STP block would show port status as BLK, but ARP incomplete suggests different issue.
✗
A static MAC address entry for the server is missing.
Why it's wrong here
The issue is with users in VLAN 100, not the server.
✓
The switch cannot resolve ARP requests because the user's host is not responding.
Why this is correct
Incomplete ARP entries indicate the switch sent ARP requests but got no reply, possibly due to host firewall or misconfiguration.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Read the scenario before looking for a memorised answer.
✗
The IRB interface for VLAN 100 is not configured with a proxy ARP.
Why it's wrong here
Proxy ARP is not needed for local subnet communication.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates may 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.
Trap categories for this question
Command / output trap
STP block would show port status as BLK, but ARP incomplete suggests different issue.
Detailed technical explanation
How to think about this question
When a Juniper switch with IRB interfaces acts as a default gateway, it uses ARP to resolve the MAC addresses of hosts in each VLAN. The 'incomplete' ARP entries mean the switch sent ARP requests (broadcast frames) but never received unicast ARP replies. This can occur if the host's NIC is in a power-saving state, the host is disconnected, or a local firewall (e.g., Windows Firewall) is blocking ICMP and ARP responses. The switch's inability to complete ARP prevents it from building the necessary forwarding table entry for inter-VLAN traffic, even though the MAC address is learned in the Ethernet switching table.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A help-desk technician troubleshoots why a newly connected PC cannot reach shared printers on the same floor. The cable is good, the switch port is active, but the PC is in VLAN 20 and the printers are in VLAN 10. The uplink trunk only allows VLAN 10. A trunk being up does not mean every VLAN crosses it.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Networking Fundamentals — This question tests Networking Fundamentals — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: 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.
What should I do if I get this JNCIA-JUNOS question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
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