mediumMultiple Choice
350-401 Practice Question: Is troubleshooting multicast video distribution…
A network engineer is troubleshooting multicast video distribution across an enterprise campus. The multicast source is connected to a switch that is the PIM Designated Router (DR) on a multi-access segment. Receivers in a different VLAN report that they are not receiving the multicast stream, although the DR shows the correct (S,G) entry. The engineer checks the RPF neighbor for the source and notices that the unicast route to the source points to a different interface than the one where the multicast stream is received. What is the most likely cause of the issue?
⚠ Common exam trap
Cisco often tests the RPF check concept by presenting a scenario where the (S,G) entry exists but traffic is not forwarded, leading candidates to incorrectly suspect IGMP snooping, RP configuration, or trunk issues, when the real problem is a unicast routing mismatch causing an RPF failure.
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 multicast stream is arriving on an interface that is not the RPF interface for the source.
PIM uses the unicast routing table to determine the Reverse Path Forwarding (RPF) interface for a multicast source. If the multicast stream arrives on an interface that is not the RPF interface (the interface the unicast route uses to reach the source), the router will perform an RPF check and fail, dropping the multicast traffic. This is the most likely cause of the issue, as the DR has the (S,G) entry but the stream is not being forwarded due to the RPF mismatch.
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 DR is not configured as the RP (Rendezvous Point).
Why it's wrong here
The DR (Designated Router) and RP (Rendezvous Point) are distinct roles in PIM. The DR is elected on a multi-access segment to forward multicast traffic to the RP or receivers, while the RP acts as the root of the shared tree. The symptom here is an RPF failure on the incoming interface, which depends solely on unicast routing toward the source, not on the DR or RP configuration. Even if the DR were correctly configured as the RP, the packet would still be dropped because the interface it arrives on is not the unicast reverse path interface for that source.
- ✓
The multicast stream is arriving on an interface that is not the RPF interface for the source.
Why this is correct
Multicast routers enforce a Reverse Path Forwarding (RPF) check: a multicast packet is accepted only if it arrives on the interface that the unicast routing table would use to reach the source IP address. If the stream arrives on a non-RPF interface, the router discards it to prevent loops and duplicate delivery. This mismatch directly matches the described symptom of multicast traffic failing to forward, making this the correct explanation.
- ✗
The switchport connected to the source is not configured as a trunk.
Why it's wrong here
Trunking is an access-layer concept used to carry multiple VLANs between switches. The interface connected to the multicast source is typically a routed interface or an access port in a specific VLAN, not a trunk port. An incorrect switchport trunk configuration would affect Layer 2 VLAN membership or inter-switch connectivity, but it would not cause an RPF failure, which is determined by the Layer 3 unicast routing table and the incoming interface for the source.
- ✗
IGMP snooping is disabled on the receiver VLAN.
Why it's wrong here
IGMP snooping operates at Layer 2 and optimizes multicast group membership by forwarding multicast frames only to ports that have IGMP report members. Disabling IGMP snooping would cause multicast frames to be flooded to all ports in the receiver VLAN, which may result in unnecessary traffic but does not affect the Layer 3 RPF check. The RPF failure is triggered by the source stream arriving on an incorrect incoming interface, independent of IGMP snooping state or receiver group membership.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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.