The answer is that the last-hop router has not yet switched to the SPT. In PIM sparse mode, the last-hop router initially joins the shared tree toward the RP, creating an (S,G) entry with the incoming interface pointing toward the RP rather than the source. This suboptimal path persists because the router has not performed the SPT switchover, which would send a join toward the source to build a direct, shortest-path tree. On the ENCOR 350-401 exam, this concept tests your understanding of the PIM sparse mode switchover mechanism and the default behavior of Cisco routers, which typically trigger the switchover after receiving the first packet. A common trap is assuming the router always uses the SPT immediately, but the shared tree is the default until the switchover threshold is met or enabled. Remember the mnemonic: "First join to RP, then switch to SPT for speed."
CCNP 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.
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?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
The last-hop router has not yet switched to the SPT.
Correct because the default behavior is to switch to the SPT after the first packet, but if disabled or delayed, the shared tree is used.
B
The RP is not configured as the DR on its segment.
Why wrong: Incorrect because the DR role is for the source segment, not for path selection.
C
The multicast source is using a different group address.
Why wrong: Incorrect because the group address is consistent; the path issue is related to tree type.
D
The last-hop router has a higher metric to the source than to the RP.
Why wrong: Incorrect because even if the metric is higher, the SPT switchover would use the source path; the issue is that the switchover hasn't occurred.
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 joins the shared tree toward the RP. After receiving the first packet, it can optionally switch to the shortest path tree (SPT) by sending a join toward the source. If the SPT switchover is disabled or delayed, the traffic continues to use the shared tree, which may be suboptimal.
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 last-hop router has not yet switched to the SPT.
Why this is correct
Correct because the default behavior is to switch to the SPT after the first packet, but if disabled or delayed, the shared tree is used.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
Static NAT maps one inside address to one outside address.
✗
The RP is not configured as the DR on its segment.
Why it's wrong here
Incorrect because the DR role is for the source segment, not for path selection.
✗
The multicast source is using a different group address.
Why it's wrong here
Incorrect because the group address is consistent; the path issue is related to tree type.
✗
The last-hop router has a higher metric to the source than to the RP.
Why it's wrong here
Incorrect because even if the metric is higher, the SPT switchover would use the source path; the issue is that the switchover hasn't occurred.
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.
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 last-hop router has not yet switched to the SPT. — In PIM sparse mode, the last-hop router initially joins the shared tree toward the RP. After receiving the first packet, it can optionally switch to the shortest path tree (SPT) by sending a join toward the source. If the SPT switchover is disabled or delayed, the traffic continues to use the shared tree, which may be suboptimal.
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.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
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