The answer is Proxy ARP, because it enables inter-VLAN routing without a default gateway configured on the hosts. When a host in VLAN 10 needs to reach a host in VLAN 20, it broadcasts an ARP request for the destination IP. The router, acting as a proxy, intercepts this request and replies with its own MAC address, provided it has a route to the target subnet. The host then sends frames directly to the router, which forwards them to the correct VLAN, effectively performing routing at Layer 2 without requiring the host to know a gateway. On the JNCIA-Junos exam, this concept tests your understanding of how Junos devices handle ARP in multi-subnet environments, often appearing as a trick question where a host pings across VLANs with no gateway set. A common trap is confusing Proxy ARP with VLAN trunking or static routing; remember that Proxy ARP works by the router impersonating the destination. Memory tip: think of the router as a “stand-in” for the remote host—it answers the ARP call so the sender never needs a gateway.
JNCIA-JUNOS Networking Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of networking fundamentals. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.
A host in VLAN 10 can ping a host in VLAN 20 without a default gateway configured. What feature is likely enabled on the router that interconnects the VLANs?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Proxy ARP
Proxy ARP allows a router to respond to ARP requests on behalf of hosts in different subnets. When a host in VLAN 10 sends an ARP request for a host in VLAN 20, the router responds with its own MAC address if it has a route to the destination. This enables the host to send traffic to the router, which then forwards it to the destination VLAN, even without a default gateway configured on the host.
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.
✗
VLAN translation is configured on the trunk
Why it's wrong here
VLAN translation changes VLAN IDs but does not affect Layer 3 forwarding.
✓
Proxy ARP
Why this is correct
Proxy ARP enables the router to reply to ARP requests for addresses in other subnets, making the host think the remote host is on the same subnet.
Related concept
Read the scenario before looking for a memorised answer.
✗
IP routing is enabled on the router
Why it's wrong here
IP routing is necessary but not sufficient; the host needs a gateway or proxy ARP.
✗
Dynamic ARP Inspection
Why it's wrong here
DAI is used to prevent ARP spoofing, not to enable routing without a gateway.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often assume IP routing alone is sufficient for inter-VLAN communication, forgetting that hosts must have a default gateway or use proxy ARP to send traffic outside their local subnet.
Detailed technical explanation
How to think about this question
Proxy ARP is defined in RFC 1027 and operates at Layer 2/3 boundary. When a host without a default gateway sends an ARP request for an IP in a different subnet, the router's proxy ARP feature (enabled by default on many platforms, including Junos) intercepts the request and replies with its own MAC address. The host then sends the packet to the router, which routes it to the destination VLAN. This behavior can be verified by checking the ARP table on the host, which will show the destination IP mapped to the router's MAC.
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: Proxy ARP — Proxy ARP allows a router to respond to ARP requests on behalf of hosts in different subnets. When a host in VLAN 10 sends an ARP request for a host in VLAN 20, the router responds with its own MAC address if it has a route to the destination. This enables the host to send traffic to the router, which then forwards it to the destination VLAN, even without a default gateway configured on the host.
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.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
This JNCIA-JUNOS 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 JNCIA-JUNOS exam.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.