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350-401 Practice Question: Is configuring multicast on a Cisco router that…
A network engineer is configuring multicast on a Cisco router that connects to a multi-access network. The engineer wants to ensure that only one router forwards multicast traffic onto the segment to avoid duplication. The engineer enables PIM on the interface. However, multicast traffic is still being duplicated on the segment. What is the most likely reason?
⚠ Common exam trap
Cisco often tests the misconception that IGMP snooping or OSPF passive interfaces cause multicast duplication, but the real trap is that candidates forget PIM DR election rules in multi-access networks, especially the tiebreaker using highest IP address when priorities are equal.
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
✓
Both routers have the same DR priority and the DR election has not completed.
In a multi-access network running PIM, the Designated Router (DR) is responsible for forwarding multicast traffic onto the segment. If both routers have the same DR priority (default 1) and the DR election has not completed or is tied, both may forward traffic, causing duplication. The DR election uses the highest IP address as a tiebreaker when priorities are equal, but if the election process is incomplete or both routers believe they are the DR, duplication occurs.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
Both routers have the same DR priority and the DR election has not completed.
Why this is correct
Correct because if the DR priority is equal, the router with the higher IP address becomes DR; if not elected, both may forward.
- ✗
The interface is configured as a passive interface under OSPF.
Why it's wrong here
Incorrect because OSPF passive interface does not affect PIM DR election.
- ✗
IGMP snooping is disabled on the switch connecting the routers.
Why it's wrong here
Incorrect because IGMP snooping is a Layer 2 feature and does not affect PIM DR behavior.
- ✗
The multicast group is in the 224.0.0.0/24 range.
Why it's wrong here
Incorrect because the group range does not affect DR election; DR election is per interface.
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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.