The answer is a missing directly connected route in the routing table, which causes the ping failure because Junos requires a route to both the source and destination addresses for ICMP echo requests to be processed. When you issue a ping with a specific source interface or address, the router must have a route in its forwarding table for that source network; without a directly connected route for 192.168.1.0/24, R1 cannot determine that the source 192.168.1.1 is locally reachable, and the packet is dropped. On the JNCIA-Junos exam, this scenario tests your understanding that a directly connected route is automatically installed only when the interface is up and has a valid IP address—if the link flaps or the interface is misconfigured, the route disappears even if the interface appears operational. A common trap is assuming that a ping will work as long as the interface is administratively up, but the routing table must explicitly contain the network. Memory tip: “No route, no ping—check the table, not the cable.”
JNCIA-JUNOS Routing Fundamentals Practice Question
This JNCIA-JUNOS practice question tests your understanding of routing 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.
A network engineer is troubleshooting a connectivity issue between two directly connected routers, R1 and R2. Both routers have IP addresses configured on their respective interfaces, and the interfaces are up. However, 'ping 192.168.1.2 source 192.168.1.1' from R1 fails. The engineer checks the routing table on R1 and sees a static route to 0.0.0.0/0 via a different next-hop, but no route for the 192.168.1.0/24 network. What is the most likely cause?
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.
The route to the directly connected network is missing from the routing table, possibly because the interface is not properly configured or the link is down.
A directly connected route appears only when the interface is up and has an IP address. The missing route indicates an interface issue despite being 'up'.
B
The ARP cache is stale and needs to be cleared.
Why wrong: ARP is not the primary issue; the route is missing.
C
The interface is not configured with an IP address.
Why wrong: The stem says interfaces are configured with IP addresses.
D
A security policy is blocking ICMP traffic between the two routers.
Why wrong: Security policies are for firewall filters, not directly connected pings unless on a zone boundary; not mentioned.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The route to the directly connected network is missing from the routing table, possibly because the interface is not properly configured or the link is down.
Option C is correct because the directly connected network must be present in the routing table for communication; if there is no route to 192.168.1.0/24, the ping source address is not considered reachable or the destination is not in the routing table. Option A is incorrect because interface state is up, so 'show interfaces terse' would show the addresses. Option B is incorrect because ARP is not a routing table entry. Option D is incorrect because a security policy is not relevant for directly connected interfaces unless zone configuration is wrong, but the issue is routing.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 route to the directly connected network is missing from the routing table, possibly because the interface is not properly configured or the link is down.
Why this is correct
A directly connected route appears only when the interface is up and has an IP address. The missing route indicates an interface issue despite being 'up'.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The ARP cache is stale and needs to be cleared.
Why it's wrong here
ARP is not the primary issue; the route is missing.
✗
The interface is not configured with an IP address.
Why it's wrong here
The stem says interfaces are configured with IP addresses.
✗
A security policy is blocking ICMP traffic between the two routers.
Why it's wrong here
Security policies are for firewall filters, not directly connected pings unless on a zone boundary; not mentioned.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
Static NAT maps one inside address to one outside address.
PAT allows many inside hosts to share one public address using ports.
Inside local and inside global describe the private and translated addresses.
NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
→Identify inside and outside interfaces first.
→Check whether the scenario needs static NAT, dynamic NAT or PAT.
→Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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.
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related JNCIA-JUNOS NAT questions on configuration and troubleshooting.
Routing Fundamentals — This question tests Routing Fundamentals — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The route to the directly connected network is missing from the routing table, possibly because the interface is not properly configured or the link is down. — Option C is correct because the directly connected network must be present in the routing table for communication; if there is no route to 192.168.1.0/24, the ping source address is not considered reachable or the destination is not in the routing table. Option A is incorrect because interface state is up, so 'show interfaces terse' would show the addresses. Option B is incorrect because ARP is not a routing table entry. Option D is incorrect because a security policy is not relevant for directly connected interfaces unless zone configuration is wrong, but the issue is routing.
What should I do if I get this JNCIA-JUNOS question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related JNCIA-JUNOS NAT questions on configuration and troubleshooting.
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?
Static NAT maps one inside address to one outside address.
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