- A
PIM sparse mode uses an explicit join model to receive multicast traffic.
Correct because in PIM sparse mode, receivers must explicitly join via PIM Join messages toward the RP.
- B
PIM sparse mode routers always use the shortest path tree (SPT) immediately after the first packet is received.
Why wrong: Incorrect because the default behavior is to use the shared tree (RP-tree) until the SPT threshold is exceeded; the SPT switchover is not immediate.
- C
PIM sparse mode builds a shared tree rooted at the rendezvous point (RP).
Correct because the RP is the root of the shared distribution tree for all sources in the group.
- D
PIM sparse mode uses a flood-and-prune mechanism to distribute multicast traffic.
Why wrong: Incorrect because flood-and-prune is characteristic of PIM dense mode, not sparse mode.
- E
PIM sparse mode requires the use of a bootstrap router (BSR) to operate.
Why wrong: Incorrect because BSR is one method for RP distribution, but PIM sparse mode can operate with static RP, Auto-RP, or BSR.
Quick Answer
The answer is that PIM sparse mode builds a shared tree rooted at the rendezvous point (RP), and it uses an explicit join model. This is correct because, unlike dense mode which floods traffic everywhere and then prunes, sparse mode requires routers to send explicit PIM Join messages toward the RP to request multicast traffic for a group. This creates a shared tree, or RP-tree, as the initial distribution path, and routers can later switch to a source-specific shortest path tree (SPT) when traffic exceeds a configured threshold. On the ENCOR 350-401 exam, this concept tests your understanding of multicast routing fundamentals—a common trap is confusing sparse mode’s explicit joins with dense mode’s flood-and-prune behavior. Remember that PIM-BSR is just one method for RP discovery, not a requirement for sparse mode operation itself. A helpful memory tip: think “Sparse = Send a Join first” to distinguish it from dense mode’s “flood first, prune later” approach.
350-401 IP Multicast Practice Question
This 350-401 practice question tests your understanding of ip multicast. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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.
Which two statements about PIM sparse mode are true? (Choose two.)
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 sparse mode uses an explicit join model to receive multicast traffic.
PIM sparse mode uses an explicit join model where routers send PIM Join messages toward the RP to join a multicast group. It builds a shared tree (RP-tree) initially, and optionally switches to a source tree (SPT) when traffic exceeds a threshold. PIM dense mode uses flood-and-prune, not sparse mode. PIM-BSR is one method for RP distribution, but not a requirement for sparse mode operation.
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.
- ✓
PIM sparse mode uses an explicit join model to receive multicast traffic.
Why this is correct
Correct because in PIM sparse mode, receivers must explicitly join via PIM Join messages toward the RP.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
PIM sparse mode routers always use the shortest path tree (SPT) immediately after the first packet is received.
Why it's wrong here
Incorrect because the default behavior is to use the shared tree (RP-tree) until the SPT threshold is exceeded; the SPT switchover is not immediate.
- ✓
PIM sparse mode builds a shared tree rooted at the rendezvous point (RP).
Why this is correct
Correct because the RP is the root of the shared distribution tree for all sources in the group.
Related concept
Static NAT maps one inside address to one outside address.
- ✗
PIM sparse mode uses a flood-and-prune mechanism to distribute multicast traffic.
Why it's wrong here
Incorrect because flood-and-prune is characteristic of PIM dense mode, not sparse mode.
- ✗
PIM sparse mode requires the use of a bootstrap router (BSR) to operate.
Why it's wrong here
Incorrect because BSR is one method for RP distribution, but PIM sparse mode can operate with static RP, Auto-RP, or BSR.
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.
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IP Multicast — study guide chapter
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FAQ
Questions learners often ask
What does this 350-401 question test?
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: PIM sparse mode uses an explicit join model to receive multicast traffic. — PIM sparse mode uses an explicit join model where routers send PIM Join messages toward the RP to join a multicast group. It builds a shared tree (RP-tree) initially, and optionally switches to a source tree (SPT) when traffic exceeds a threshold. PIM dense mode uses flood-and-prune, not sparse mode. PIM-BSR is one method for RP distribution, but not a requirement for sparse mode operation.
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.
What is the key concept behind this question?
Static NAT maps one inside address to one outside address.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
Same concept, more angles
1 more ways this is tested on 350-401
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. Which two statements about PIM sparse mode (PIM-SM) are true? (Choose two.)
medium- ✓ A.PIM-SM uses a pull model where receivers explicitly join the multicast group.
- ✓ B.In PIM-SM, the rendezvous point (RP) is the root of the shared tree.
- C.PIM-SM automatically switches to the shortest path tree (SPT) immediately after the first multicast packet is received.
- D.PIM-SM requires all routers in the domain to be configured with the same RP address.
- E.PIM-SM supports only one RP per multicast group.
Why A: PIM-SM uses a pull model where receivers join the shared tree via the RP, and the RP is the root of the shared tree. The SPT switchover occurs after the first packet arrives via the shared tree.
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Last reviewed: Jun 18, 2026
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