mediumMultiple Choice
CCNP Practice Question: An engineer is troubleshooting multicast…
An engineer is troubleshooting multicast performance issues. The network uses PIM sparse mode with a static RP. The engineer notices that the multicast traffic from a source to a group is taking a suboptimal path, causing high latency. The engineer checks the multicast routing table on the last-hop router and sees that the (S,G) entry has an incoming interface that is not the shortest path to the source. What is the most likely reason for this suboptimal path?
⚠ Common exam trap
Cisco often tests the misconception that a suboptimal path is caused by RP placement or metric issues, when in fact the core concept is the SPT switchover mechanism in PIM sparse mode.
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 last-hop router has not yet switched to the SPT.
In PIM sparse mode, the last-hop router initially receives multicast traffic via the shared tree (RP) and then switches to the shortest path tree (SPT) toward the source once the traffic rate exceeds a threshold (default 0 kbps in Cisco IOS). If the (S,G) entry shows an incoming interface that is not the shortest path to the source, it indicates the router has not yet performed the SPT switchover, so it is still using the RP-based path, which may be suboptimal.
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 last-hop router has not yet switched to the SPT.
Why this is correct
In Protocol Independent Multicast sparse-mode, the last-hop router initially joins the shared tree toward the RP. After receiving the first data packet from the source via the RP, it should immediately send an (S,G) join toward the source to switch to the shortest path tree. If that switchover hasn't occurred—commonly because the spt-threshold is set to infinity—traffic continues to be forwarded along the shared tree, explaining why the packet path still goes through the RP.
- ✗
The RP is not configured as the DR on its segment.
Why it's wrong here
The Designated Router (DR) on a segment is responsible only for sending PIM register messages on behalf of a directly connected source, not for selecting the tree used by other routers. The RP's DR status on its own segment has no bearing on whether last-hop routers use the SPT or the shared tree. In this scenario, the issue is purely a tree-switching delay, not any DR misconfiguration on the RP's link.
- ✗
The multicast source is using a different group address.
Why it's wrong here
If the source were sending to a different group address, the last-hop router wouldn't receive those packets at all—they'd be dropped because the router has no group state for that other group. The fact that traffic is flowing at all, albeit via the RP, proves the group address is correctly matched. This option incorrectly conflates the tree type with group addressing; the group address is consistent, and the actual problem is the absence of an SPT switchover.
- ✗
The last-hop router has a higher metric to the source than to the RP.
Why it's wrong here
The SPT switchover in PIM-SM is triggered by the first packet arriving at the last-hop router, or by the spt-threshold configuration—not by any comparison of unicast metrics to the source versus the RP. Even if the metric to the source is higher, PIM defaults to switching to the SPT anyway, and a higher metric would only delay the switch if a policy like spt-threshold were in place. Thus, a metric difference alone cannot explain why the traffic remains on the shared tree.
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JA
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.