- A
The uRPF strict mode check fails because the route to the source's network points out a different interface than the one on which the multicast packet was received.
Correct because multicast uRPF requires the source address to be reachable via the receiving interface; otherwise, the packet is dropped to prevent spoofing.
- B
The ACL is blocking the multicast traffic because the source address is not permitted.
Why wrong: Incorrect because the ACL permits the group address, and the source address is not typically filtered in multicast ACLs unless specified.
- C
The multicast routing is not enabled globally.
Why wrong: Incorrect because the router is receiving multicast traffic, indicating multicast routing is enabled.
- D
The downstream receivers are not in the multicast group.
Why wrong: Incorrect because the issue is with forwarding, not group membership.
Quick Answer
The answer is the uRPF strict mode check failing because the route to the source’s network points out a different interface than the one receiving the multicast packet. This occurs because strict unicast RPF verifies that the source address of an incoming IPv6 packet is reachable via the exact interface on which it arrived; if the routing table shows a different outgoing interface for that source, the packet is silently dropped before any multicast forwarding can happen. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how IPv6 unicast RPF failure symptoms can masquerade as a multicast issue—a common trap is to blame the ACL or multicast configuration when the real culprit is the reverse path check. Remember that strict mode demands symmetry: the packet’s source must be reachable back out the same door it came in. A helpful memory tip is “strict checks the door, not the destination”—if the return route leaves through a different door, the packet is dropped at the doorstep.
300-410 IPv6 Traffic Filtering and uRPF Practice Question
This 300-410 practice question tests your understanding of ipv6 traffic filtering and urpf. 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 an IPv6 multicast issue on a router. The router is receiving multicast traffic from a source, but the traffic is not being forwarded to downstream receivers. The engineer checks the interface and finds an inbound IPv6 ACL that permits only certain multicast groups. The ACL permits the group address FF1E::1, but the source address is 2001:db8:1::1. The engineer also notices that uRPF is enabled on the interface in strict mode. The router has a route to the source's network via a different interface. What is the most likely cause of the multicast forwarding failure?
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 uRPF strict mode check fails because the route to the source's network points out a different interface than the one on which the multicast packet was received.
The correct answer is A. With uRPF strict mode enabled, the router checks that the source address of the incoming packet is reachable via the same interface on which the packet was received. Since the route to 2001:db8:1::1 points out a different interface, the uRPF check fails and the packet is dropped before any multicast forwarding can occur. This explains why the multicast traffic is not forwarded to downstream receivers despite the ACL permitting the group address.
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 uRPF strict mode check fails because the route to the source's network points out a different interface than the one on which the multicast packet was received.
Why this is correct
Correct because multicast uRPF requires the source address to be reachable via the receiving interface; otherwise, the packet is dropped to prevent spoofing.
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 ACL is blocking the multicast traffic because the source address is not permitted.
Why it's wrong here
Incorrect because the ACL permits the group address, and the source address is not typically filtered in multicast ACLs unless specified.
- ✗
The multicast routing is not enabled globally.
Why it's wrong here
Incorrect because the router is receiving multicast traffic, indicating multicast routing is enabled.
- ✗
The downstream receivers are not in the multicast group.
Why it's wrong here
Incorrect because the issue is with forwarding, not group membership.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates often focus on the ACL or multicast configuration and overlook the uRPF strict mode check, which silently drops packets before multicast forwarding logic is applied.
Detailed technical explanation
How to think about this question
uRPF (unicast Reverse Path Forwarding) in strict mode leverages the FIB to verify that the source address of an incoming packet is reachable via the ingress interface. For IPv6 multicast, this check is performed before any multicast-specific processing (e.g., PIM, MLD). In real-world scenarios, asymmetric routing (where the return path to the source uses a different interface) commonly causes uRPF strict mode to drop legitimate multicast traffic, often requiring a switch to loose mode or an interface-specific exception.
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 security administrator must allow nursing staff to reach a patient records server while blocking access from the guest Wi-Fi VLAN. After applying an extended ACL, traffic is still blocked from nursing workstations. The ACL was applied outbound instead of inbound on the wrong interface. Questions like this test ACL direction and placement rules.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
- →
IPv6 Traffic Filtering and uRPF — study guide chapter
Learn the concepts, then practise the questions
- →
IPv6 Traffic Filtering and uRPF practice questions
Targeted practice on this topic area only
- →
All 300-410 questions
2,152 questions across all exam domains
- →
Cisco CCNP ENARSI 300-410 study guide
Full concept coverage aligned to exam objectives
- →
300-410 practice test guide
How to use practice tests most effectively before exam day
Related practice questions
Related 300-410 practice-question pages
Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.
Layer 3 Technologies practice questions
Practise 300-410 questions linked to Layer 3 Technologies.
EIGRP Troubleshooting practice questions
Practise 300-410 questions linked to EIGRP Troubleshooting.
OSPF Troubleshooting (v2/v3) practice questions
Practise 300-410 questions linked to OSPF Troubleshooting (v2/v3).
BGP Troubleshooting practice questions
Practise 300-410 questions linked to BGP Troubleshooting.
Route Redistribution practice questions
Practise 300-410 questions linked to Route Redistribution.
Policy-Based Routing (PBR) practice questions
Practise 300-410 questions linked to Policy-Based Routing (PBR).
VRF-Lite practice questions
Practise 300-410 questions linked to VRF-Lite.
Route Maps and Route Filtering practice questions
Practise 300-410 questions linked to Route Maps and Route Filtering.
Administrative Distance practice questions
Practise 300-410 questions linked to Administrative Distance.
Route Summarization practice questions
Practise 300-410 questions linked to Route Summarization.
Bidirectional Forwarding Detection (BFD) practice questions
Practise 300-410 questions linked to Bidirectional Forwarding Detection (BFD).
VPN Technologies practice questions
Practise 300-410 questions linked to VPN Technologies.
Practice this exam
Start a free 300-410 practice session
Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.
FAQ
Questions learners often ask
What does this 300-410 question test?
IPv6 Traffic Filtering and uRPF — This question tests IPv6 Traffic Filtering and uRPF — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The uRPF strict mode check fails because the route to the source's network points out a different interface than the one on which the multicast packet was received. — The correct answer is A. With uRPF strict mode enabled, the router checks that the source address of the incoming packet is reachable via the same interface on which the packet was received. Since the route to 2001:db8:1::1 points out a different interface, the uRPF check fails and the packet is dropped before any multicast forwarding can occur. This explains why the multicast traffic is not forwarded to downstream receivers despite the ACL permitting the group address.
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.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more ways this is tested on 300-410
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which THREE symptoms indicate that IPv6 unicast RPF is misconfigured or failing on an interface? (Choose THREE.)
hard- ✓ A.Traffic from a valid source IP is being dropped on the interface.
- ✓ B.The router logs 'IPv6 unicast RPF drop' messages.
- C.The command 'ipv6 verify unicast source reachable-via any' is present in the running config.
- D.High CPU utilization is observed due to RPF processing.
- ✓ E.The 'show ipv6 interface' output shows an increasing 'RPF drops' counter.
Why A: uRPF drops packets if the source address is not reachable via the incoming interface. A syslog message about uRPF drops confirms this. The 'ipv6 verify unicast source reachable-via any' command is a configuration command, not a symptom. High CPU due to RPF checks is not typical; RPF is done in hardware (CEF). An increase in the 'ipv6_input' drop counter for 'RPF' indicates drops.
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.
Question Discussion
Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.
Sign in to join the discussion.