- A
Asymmetric routing is in use, and the return route for the source IP points to a different interface.
Strict uRPF requires that the best route to the source IP address points back to the same interface on which the packet was received. If asymmetric routing is present, the return path may be via a different interface, causing strict uRPF to drop the packet.
- B
The 'allow-default' option is not enabled, so default routes are not considered.
Why wrong: The 'allow-default' option is used in loose mode to allow packets with source IPs that match a default route. In strict mode, default routes are not used for the check unless explicitly configured, but the question states the route exists, so this is not the issue.
- C
The 'ip verify unicast source reachable-via any' command was used instead of 'rx'
Why wrong: Using 'any' enables loose mode, not strict mode. The question states strict mode is configured, so this is not the case.
- D
The customer subnet is a summary route, and the more specific route is missing.
Why wrong: uRPF uses the best route in the routing table; if a summary route exists, it is used for the check. Missing more specific routes would not cause drops if the summary is present.
300-410 MPLS Operations Practice Question
This 300-410 practice question tests your understanding of mpls operations. 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 unicast Reverse Path Forwarding (uRPF) in strict mode on an interface. After the configuration, legitimate traffic from a customer network is being dropped. The engineer confirms that the route for the customer subnet exists in the routing table and points to the correct interface. 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 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
Asymmetric routing is in use, and the return route for the source IP points to a different interface.
Strict uRPF checks that the source IP address of an incoming packet has a route in the routing table that points back to the same interface. If the customer network uses asymmetric routing (i.e., traffic comes in one interface but the return route points out a different interface), strict uRPF will drop the traffic. The edge case is that even if the route exists, if it does not point to the incoming interface, the packet is dropped.
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.
- ✓
Asymmetric routing is in use, and the return route for the source IP points to a different interface.
Why this is correct
Strict uRPF requires that the best route to the source IP address points back to the same interface on which the packet was received. If asymmetric routing is present, the return path may be via a different interface, causing strict uRPF to drop the packet.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
CIDR notation defines the prefix length.
- ✗
The 'allow-default' option is not enabled, so default routes are not considered.
Why it's wrong here
The 'allow-default' option is used in loose mode to allow packets with source IPs that match a default route. In strict mode, default routes are not used for the check unless explicitly configured, but the question states the route exists, so this is not the issue.
- ✗
The 'ip verify unicast source reachable-via any' command was used instead of 'rx'
Why it's wrong here
Using 'any' enables loose mode, not strict mode. The question states strict mode is configured, so this is not the case.
- ✗
The customer subnet is a summary route, and the more specific route is missing.
Why it's wrong here
uRPF uses the best route in the routing table; if a summary route exists, it is used for the check. Missing more specific routes would not cause drops if the summary is present.
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 300-410 subnetting questions on CIDR, address ranges, and subnet selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
MPLS Operations — This question tests MPLS Operations — CIDR notation defines the prefix length..
What is the correct answer to this question?
The correct answer is: Asymmetric routing is in use, and the return route for the source IP points to a different interface. — Strict uRPF checks that the source IP address of an incoming packet has a route in the routing table that points back to the same interface. If the customer network uses asymmetric routing (i.e., traffic comes in one interface but the return route points out a different interface), strict uRPF will drop the traffic. The edge case is that even if the route exists, if it does not point to the incoming interface, the packet is dropped.
What should I do if I get this 300-410 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 300-410 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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Last reviewed: Jun 18, 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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