The correct answer is to configure 'family inet' on the tunnel interface with the command set interfaces gr-0/0/0 unit 0 family inet. This is required because a GRE tunnel interface in Junos is a virtual point-to-point link that must have an explicitly assigned protocol family to carry traffic; without the 'family inet' statement, the interface lacks an IPv4 address family, preventing the Junos kernel from installing routes or forwarding packets through the tunnel, leaving it operationally down. On the JNCIA-Junos exam, this question tests your understanding that tunnel interfaces are not automatically assigned a protocol family—a common trap is assuming that setting the tunnel source and destination alone is sufficient. The exam often pairs this with the fact that the tunnel source references loopback 0 and the destination is 192.0.2.1, but neither of those configurations activates the interface for IPv4 traffic. A helpful memory tip: think of "family inet" as the key that unlocks the tunnel door—without it, no packets can enter or exit.
JNCIA-JUNOS Junos Configuration Basics Practice Question
This JNCIA-JUNOS practice question tests your understanding of junos configuration basics. 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 network administrator is configuring a GRE tunnel on a Juniper device. The tunnel source is loopback 0 and destination is 192.0.2.1. Which additional configuration is necessary on the tunnel interface for the tunnel to become operational?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
set interfaces gr-0/0/0 unit 0 family inet
Option D is correct because a GRE tunnel interface on Junos requires the 'family inet' statement to be configured under the unit to enable IPv4 traffic forwarding. Without this, the tunnel interface will not have an IPv4 protocol family, and the device will not be able to route packets into or out of the tunnel, leaving it operationally down.
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.
✗
set interfaces gr-0/0/0 unit 0 mtu 1476
Why it's wrong here
MTU configuration is optional and not strictly required for tunnel operation.
✗
set interfaces gr-0/0/0 unit 0 multicast
Why it's wrong here
Multicast is not required for basic tunnel operation.
✗
set interfaces gr-0/0/0 unit 0 family inet6
Why it's wrong here
Not required for IPv4 GRE tunnel.
✓
set interfaces gr-0/0/0 unit 0 family inet
Why this is correct
Enables IPv4 on the tunnel interface, necessary for routing.
Related concept
Read the scenario before looking for a memorised answer.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates assume GRE tunnels automatically support IPv4 traffic on Junos, similar to Cisco IOS, but Junos requires an explicit 'family inet' configuration to activate the protocol family on the tunnel interface.
Detailed technical explanation
How to think about this question
GRE tunnels in Junos are modeled as logical interfaces under a physical GRE PIC (e.g., gr-0/0/0). The 'family inet' configuration activates the IPv4 protocol stack on the tunnel interface, allowing the kernel to assign an IPv4 address and install routes via the tunnel. Without this family statement, the interface remains in a 'down' state for IPv4 traffic, even if the physical and tunnel endpoints are reachable. This is a common pitfall for engineers migrating from Cisco IOS, where GRE tunnels implicitly support IPv4 without an explicit family declaration.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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.
Junos Configuration Basics — This question tests Junos Configuration Basics — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: set interfaces gr-0/0/0 unit 0 family inet — Option D is correct because a GRE tunnel interface on Junos requires the 'family inet' statement to be configured under the unit to enable IPv4 traffic forwarding. Without this, the tunnel interface will not have an IPv4 protocol family, and the device will not be able to route packets into or out of the tunnel, leaving it operationally down.
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.
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Question Discussion
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