The answer is that strict mode uRPF drops the traffic because a default route does not satisfy its requirement for a specific, exact-match route for the source IP. Strict mode uRPF performs two checks: it verifies that the source IP address has a matching entry in the routing table, and that the best return path to that source uses the same interface on which the packet arrived. A default route (0.0.0.0/0) is a catch-all entry, not a specific route, so the router cannot find an exact match for the customer’s source IP and drops the packet. On the Cisco CCNP ENARSI 300-410 exam, this is a classic trap—candidates often assume a default route will work, but strict mode demands a precise route like a connected or static entry. A helpful memory tip is “Strict is specific; a default is too broad.”
300-410 IPv4 Access Control Lists Practice Question
This 300-410 practice question tests your understanding of ipv4 access control lists. 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.
An engineer configures unicast Reverse Path Forwarding (uRPF) in strict mode on an interface connected to a customer network. The customer has a default route pointing to the router. After enabling uRPF, the router drops traffic from the customer that has a source IP address that is not in the routing table as a directly connected or static route. What 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 the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Strict mode uRPF requires a specific route for the source IP; a default route is not sufficient.
Strict mode uRPF verifies that the source IP address of an incoming packet matches a specific route in the routing table, and that the interface used to reach that source IP is the same as the incoming interface. A default route (0.0.0.0/0) is a catch-all entry and does not provide a specific, directly connected or static route for the customer's source IP. Therefore, the router drops the traffic because it cannot find an exact match for the source IP in the routing table, which is a fundamental requirement of strict mode.
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.
✓
Strict mode uRPF requires a specific route for the source IP; a default route is not sufficient.
Why this is correct
Strict mode checks for a matching route with the same incoming interface; a default route does not provide a specific interface match.
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 customer's default route causes asymmetric routing, which breaks strict mode.
Why it's wrong here
Asymmetric routing is a problem for strict mode, but the issue here is missing route for the source IP.
✗
The uRPF mode should be loose mode to allow the default route.
Why it's wrong here
Loose mode only checks that a route exists, not the interface, but the question specifies strict mode.
✗
The router has an ACL that is blocking the traffic before uRPF checks.
Why it's wrong here
uRPF is applied before ACLs, so ACL is not the cause.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the misconception that a default route satisfies strict mode uRPF requirements, but the trap is that strict mode demands a specific route (not a default) for the source IP, and the incoming interface must match the route's outgoing interface.
Detailed technical explanation
How to think about this question
Strict mode uRPF, as defined in RFC 3704, performs a Forwarding Information Base (FIB) lookup on the source IP address and requires a matching route with the same incoming interface. A default route is stored in the FIB but does not specify an interface for the source IP; it only provides a next-hop for destination IPs. In real-world scenarios, this is common when a customer edge router uses a default route to an ISP, and the ISP must either use loose mode uRPF or configure a specific static route for the customer's prefix to avoid dropping legitimate traffic.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
IPv4 Access Control Lists — This question tests IPv4 Access Control Lists — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: Strict mode uRPF requires a specific route for the source IP; a default route is not sufficient. — Strict mode uRPF verifies that the source IP address of an incoming packet matches a specific route in the routing table, and that the interface used to reach that source IP is the same as the incoming interface. A default route (0.0.0.0/0) is a catch-all entry and does not provide a specific, directly connected or static route for the customer's source IP. Therefore, the router drops the traffic because it cannot find an exact match for the source IP in the routing table, which is a fundamental requirement of strict mode.
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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Question Discussion
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