- A
The RPF check ensures that multicast packets are forwarded only if they arrive on the interface that the router would use to send unicast traffic back to the source.
Correct because the RPF check uses the unicast routing table to determine the expected incoming interface for the source.
- B
If the RPF check fails, the multicast packet is dropped by the router.
Correct because a failed RPF check indicates a potential routing loop, so the packet is discarded.
- C
The RPF check is performed only on the first packet of a multicast stream to determine the forwarding path.
Why wrong: Incorrect because the RPF check is performed on every incoming multicast packet to maintain loop-free forwarding.
- D
The RPF check relies solely on the multicast routing table (MRIB) to determine the incoming interface.
Why wrong: Incorrect because the RPF check uses the unicast routing table (RIB), not the multicast routing table (MRIB).
- E
A multicast packet can fail the RPF check even if the unicast route to the source exists, if the packet arrives on a different interface than the one used for unicast return traffic.
Correct because the RPF check compares the arrival interface with the unicast route's outgoing interface; a mismatch causes failure.
350-401 IP Multicast Practice Question
This 350-401 practice question tests your understanding of ip multicast. 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.
Which three statements about RPF check in IP multicast are true? (Choose three.)
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 RPF check ensures that multicast packets are forwarded only if they arrive on the interface that the router would use to send unicast traffic back to the source.
The RPF check is a fundamental multicast forwarding mechanism that verifies the source address of incoming multicast packets against the unicast routing table. A packet passes RPF if it arrives on the interface that the router would use to reach the source. If the RPF check fails, the packet is dropped to prevent loops. RPF is performed on every multicast packet, not just the first one. The RPF check is independent of the multicast routing protocol; it uses the unicast routing table. RPF failure can occur even with a correct unicast route if the packet arrives on a different interface.
Key principle: NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 RPF check ensures that multicast packets are forwarded only if they arrive on the interface that the router would use to send unicast traffic back to the source.
Why this is correct
Correct because the RPF check uses the unicast routing table to determine the expected incoming interface for the source.
Related concept
Static NAT maps one inside address to one outside address.
- ✓
If the RPF check fails, the multicast packet is dropped by the router.
Why this is correct
Correct because a failed RPF check indicates a potential routing loop, so the packet is discarded.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
The RPF check is performed only on the first packet of a multicast stream to determine the forwarding path.
Why it's wrong here
Incorrect because the RPF check is performed on every incoming multicast packet to maintain loop-free forwarding.
- ✗
The RPF check relies solely on the multicast routing table (MRIB) to determine the incoming interface.
Why it's wrong here
Incorrect because the RPF check uses the unicast routing table (RIB), not the multicast routing table (MRIB).
- ✓
A multicast packet can fail the RPF check even if the unicast route to the source exists, if the packet arrives on a different interface than the one used for unicast return traffic.
Why this is correct
Correct because the RPF check compares the arrival interface with the unicast route's outgoing interface; a mismatch causes failure.
Related concept
Static NAT maps one inside address to one outside address.
Common exam traps
Common exam trap: NAT rules depend on direction and matching traffic
NAT is not only about the public address. The inside/outside interface roles and the ACL or rule that matches traffic are just as important.
Detailed technical explanation
How to think about this question
NAT questions usually test address translation, overload/PAT behaviour, static mappings and whether the right traffic is being translated. Read the interface direction and address terms carefully.
KKey Concepts to Remember
- Static NAT maps one inside address to one outside address.
- PAT allows many inside hosts to share one public address using ports.
- Inside local and inside global describe the private and translated addresses.
- NAT ACLs identify traffic for translation, not always security filtering.
TExam Day Tips
- Identify inside and outside interfaces first.
- Check whether the scenario needs static NAT, dynamic NAT or PAT.
- Do not confuse NAT matching ACLs with normal packet-filtering intent.
Key takeaway
NAT direction and interface roles matter as much as the IP address mapping. Inside/outside designation controls which traffic is translated.
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 the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-401 NAT questions on configuration and troubleshooting.
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FAQ
Questions learners often ask
What does this 350-401 question test?
IP Multicast — This question tests IP Multicast — Static NAT maps one inside address to one outside address..
What is the correct answer to this question?
The correct answer is: The RPF check ensures that multicast packets are forwarded only if they arrive on the interface that the router would use to send unicast traffic back to the source. — The RPF check is a fundamental multicast forwarding mechanism that verifies the source address of incoming multicast packets against the unicast routing table. A packet passes RPF if it arrives on the interface that the router would use to reach the source. If the RPF check fails, the packet is dropped to prevent loops. RPF is performed on every multicast packet, not just the first one. The RPF check is independent of the multicast routing protocol; it uses the unicast routing table. RPF failure can occur even with a correct unicast route if the packet arrives on a different interface.
What should I do if I get this 350-401 question wrong?
Review the four NAT address types (inside local, inside global, outside local, outside global), PAT port overload, and static vs dynamic NAT use cases. Then practise related 350-401 NAT questions on configuration and troubleshooting.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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Last reviewed: Jun 18, 2026
This 350-401 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 350-401 exam.
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