- A
The router has a default route pointing to a different interface, causing the source IP to fail the reachability check.
uRPF strict mode requires that the best route to the source IP points back out the same interface. If a default route points elsewhere, the check fails, and packets are dropped.
- B
The directly connected network is not in the routing table because the interface is down.
Why wrong: If the interface is down, the network would not be directly connected, but the problem states it is directly connected.
- C
The uRPF command is missing the 'allow-default' option, which is required for directly connected networks.
Why wrong: The 'allow-default' option allows the use of a default route for the reachability check, but it is not required for directly connected networks. It is used when the source IP is reachable via a default route.
- D
The router has a static route for the source network with a different next-hop, causing the interface check to fail.
Why wrong: If the static route points to a different interface, the check would fail, but the problem states it is directly connected, so the route should point out the same interface.
Quick Answer
The answer is that a default route pointing out a different interface is the most likely cause of uRPF strict mode dropping traffic from a directly connected network. In strict mode, the router performs a reverse path lookup on the source IP of each incoming packet, requiring that the best return route points back out the exact interface where the packet arrived. If a default route exists via another interface, the router will use that default for the source IP’s reachability check instead of a more specific route, causing the check to fail even though the traffic is legitimate and no asymmetric routing is present. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding that uRPF strict mode relies on the Forwarding Information Base (FIB) for its lookup, not just connected routes, and a default route can override a directly connected network’s expected path. A common trap is assuming that directly connected networks always pass the check, but the default route creates a mismatch. Remember: strict mode demands a specific route back through the ingress interface—if your only route home is a default out another door, the packet gets dropped.
300-410 Device Access Control Practice Question
This 300-410 practice question tests your understanding of device access control. 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.
An engineer configures uRPF strict mode on an interface. After configuration, legitimate traffic from a directly connected network is dropped. The network is connected via a single link, and there is no asymmetric routing. Which is the most likely explanation?
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 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 router has a default route pointing to a different interface, causing the source IP to fail the reachability check.
In strict uRPF mode, the router checks that the source IP address of an incoming packet is reachable via the same interface on which the packet arrived. If the router has a default route pointing out a different interface, the source IP of a directly connected network may not have a specific route back through the receiving interface, causing the reachability check to fail and legitimate traffic to be dropped.
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.
- ✓
The router has a default route pointing to a different interface, causing the source IP to fail the reachability check.
Why this is correct
uRPF strict mode requires that the best route to the source IP points back out the same interface. If a default route points elsewhere, the check fails, and packets are dropped.
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 directly connected network is not in the routing table because the interface is down.
Why it's wrong here
If the interface is down, the network would not be directly connected, but the problem states it is directly connected.
- ✗
The uRPF command is missing the 'allow-default' option, which is required for directly connected networks.
Why it's wrong here
The 'allow-default' option allows the use of a default route for the reachability check, but it is not required for directly connected networks. It is used when the source IP is reachable via a default route.
- ✗
The router has a static route for the source network with a different next-hop, causing the interface check to fail.
Why it's wrong here
If the static route points to a different interface, the check would fail, but the problem states it is directly connected, so the route should point out the same interface.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that uRPF strict mode only checks for a route existence, when in fact it also requires the incoming interface to match the route's outgoing interface, and a default route can interfere with this check.
Detailed technical explanation
How to think about this question
Strict uRPF (RFC 3704) performs a two-part check: first, the source IP must have a matching route in the FIB, and second, that route's outgoing interface must match the ingress interface. A default route is often a less specific route; if it points out a different interface, the source IP of a directly connected network may be matched by the default route instead of the connected route, causing the interface check to fail. In real-world scenarios, this commonly occurs when a router has a default route pointing to a WAN interface while receiving traffic from a LAN interface that is directly connected.
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.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Device Access Control — This question tests Device Access Control — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The router has a default route pointing to a different interface, causing the source IP to fail the reachability check. — In strict uRPF mode, the router checks that the source IP address of an incoming packet is reachable via the same interface on which the packet arrived. If the router has a default route pointing out a different interface, the source IP of a directly connected network may not have a specific route back through the receiving interface, causing the reachability check to fail and legitimate traffic to be dropped.
What should I do if I get this 300-410 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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Last reviewed: Jun 24, 2026
This 300-410 practice question is part of Courseiva's free Cisco 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 300-410 exam.
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