hardMultiple Choice
350-401 Practice Question: Is configuring PIM sparse mode in a network that…
A network engineer is configuring PIM sparse mode in a network that uses a Bootstrap Router (BSR) for RP discovery. The engineer has configured a candidate BSR and candidate RPs. However, some routers in the network are not learning the RP set. The engineer checks the BSR and sees that it is receiving candidate RP advertisements, but the BSR messages are not being forwarded to all routers. What is the most likely cause?
⚠ Common exam trap
350-401 often tests whether candidates blame BSR priority or OSPF area mismatches when the real issue is that PIM must be enabled on every transit interface for BSR messages to flood correctly.
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
✓
PIM is not enabled on all interfaces between the BSR and the other routers.
BSR messages are PIM protocol messages that must traverse every router between the BSR and the receivers. If PIM is not enabled on all transit interfaces, those routers will not forward BSR messages, so downstream routers never learn the RP set. Enabling PIM on all relevant interfaces restores BSR flooding and RP discovery.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
PIM is not enabled on all interfaces between the BSR and the other routers.
Why this is correct
BSR messages are PIM protocol packets forwarded hop by hop, so every transit interface between the BSR and downstream routers must have PIM enabled. Without PIM on those links, the bootstrap messages are dropped and routers never receive the RP set, even though the BSR itself receives candidate RP advertisements.
- ✗
The candidate BSR priority is set too low.
Why it's wrong here
BSR priority only determines which candidate wins the BSR election; it does not govern whether the elected BSR's bootstrap messages are forwarded. Lowering priority is tempting when redundancy is desired, but the forwarding failure stems from the BSR messages not propagating across all PIM-enabled interfaces.
- ✗
The candidate RPs are not in the same OSPF area as the BSR.
Why it's wrong here
BSR messages flood hop-by-hop within the PIM domain and are not constrained by OSPF area boundaries, provided unicast reachability exists. Placing candidate RPs in a different OSPF area is valid design, so this cannot stop bootstrap forwarding. The actual cause is typically a missing or filtered PIM neighbour.
- ✗
The BSR is not configured as a candidate RP.
Why it's wrong here
A candidate RP role is unrelated to forwarding BSR messages; the BSR floods these hop-by-hop across all PIM-enabled interfaces regardless of RP candidacy. Configuring the BSR as a candidate RP is common practice for redundancy, but it does not affect whether bootstrap messages reach every router.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
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.