The answer is that the return route for the source IP points to a different interface than the one where the packet arrived. uRPF strict mode performs a reverse path lookup on the source address, and if the best return route does not exit through the exact ingress interface, the router considers the packet spoofed and drops it—even if the source is legitimate. This behavior is why uRPF strict mode drops legitimate traffic when asymmetric routing or unequal-cost paths exist, as the router cannot verify the source’s reachability back through the receiving interface. On the ENCOR 350-401 exam, this concept tests your understanding of how uRPF validates source addresses against the FIB, and a common trap is assuming that simply having a route in the table is sufficient. Remember the memory tip: “Strict checks the stick—if the return path doesn’t match the entry point, the packet gets the flick.”
CCNP Infrastructure Security Practice Question
This 350-401 practice question tests your understanding of infrastructure security. Match the stated requirement to the specific cloud service, access model, or configuration option — many options are valid in isolation but not for this scenario. 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 configuring uRPF (unicast Reverse Path Forwarding) on a Cisco router to prevent spoofed IP traffic. The engineer enables uRPF in strict mode on the ingress interface connected to the internal network. After enabling uRPF, legitimate traffic from internal hosts is being dropped. The engineer checks the routing table and sees that the routes for the internal subnets are present. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The return route for the source IP points to a different interface than the one where the packet arrived.
uRPF strict mode checks that the source IP address of a packet has a route back to the same interface. If the router has multiple equal-cost paths or if the return route points to a different interface, uRPF drops the packet. Option A is correct because the return path must match the ingress interface. Option B is incorrect because uRPF does not check the destination. Option C is incorrect because uRPF does not require a default route. Option D is incorrect because uRPF works with static routes.
Key principle: Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 return route for the source IP points to a different interface than the one where the packet arrived.
Why this is correct
Correct because strict uRPF requires the source IP to have a route back out the same interface.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
uRPF is checking the destination IP address, which is not reachable.
Why it's wrong here
Incorrect because uRPF checks the source IP, not the destination.
✗
The router does not have a default route, so uRPF drops all traffic.
Why it's wrong here
Incorrect because uRPF only checks for a matching route, not a default route.
✗
uRPF cannot be used with static routes; it requires a dynamic routing protocol.
Why it's wrong here
Incorrect because uRPF works with any type of route.
Common exam traps
Common exam trap: usable hosts are not the same as total addresses
Subnetting questions often tempt you into counting all addresses. In normal IPv4 subnets, the network and broadcast addresses are not usable host addresses.
Detailed technical explanation
How to think about this question
Subnetting questions test whether you can identify the network, broadcast address, usable range, mask and correct subnet. Slow down enough to calculate the block size correctly.
KKey Concepts to Remember
CIDR notation defines the prefix length.
Block size helps identify subnet boundaries.
Network and broadcast addresses are not usable hosts in normal IPv4 subnets.
The required host count determines the smallest suitable subnet.
TExam Day Tips
→Write the block size before choosing the subnet.
→Check whether the question asks for hosts, subnets or a specific address range.
→Do not confuse /24, /25, /26 and /27 host counts.
Key takeaway
Count usable hosts — not total addresses — and remember that the network and broadcast addresses are not available to hosts in standard IPv4 subnets.
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 block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 350-401 subnetting questions on CIDR, address ranges, and subnet selection.
Infrastructure Security — This question tests Infrastructure Security — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: The return route for the source IP points to a different interface than the one where the packet arrived. — uRPF strict mode checks that the source IP address of a packet has a route back to the same interface. If the router has multiple equal-cost paths or if the return route points to a different interface, uRPF drops the packet. Option A is correct because the return path must match the ingress interface. Option B is incorrect because uRPF does not check the destination. Option C is incorrect because uRPF does not require a default route. Option D is incorrect because uRPF works with static routes.
What should I do if I get this 350-401 question wrong?
Review block sizes, usable host formulas (2^n − 2), and how to find network and broadcast addresses for /24 through /30. Then practise related 350-401 subnetting questions on CIDR, address ranges, and subnet selection.
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?
CIDR notation defines the prefix length.
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