Quick Answer
The answer is the first-hop router. In PIM sparse mode, the router directly connected to the multicast source is responsible for encapsulating the source’s multicast traffic in unicast PIM register messages and forwarding them to the rendezvous point (RP). This encapsulation is necessary because the RP initially has no knowledge of the source; the first-hop router’s register message triggers the RP to join the source-specific tree (SPT) toward the source, establishing the forwarding path. On the Cisco DCCOR and CCNP Data Center Core 350-601 exam, this concept tests your understanding of the PIM sparse mode control-plane flow, often appearing in questions about RP discovery or multicast forwarding. A common trap is confusing the first-hop router with the designated router (DR) or the RP itself—remember, the DR handles shared-tree joins from receivers, while the first-hop router handles source registration. Memory tip: “First-hop sends the register—RP gets the message.”
This 350-601 practice question tests your understanding of network. 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.
A multicast application requires that all receivers join the same group using PIM sparse mode. Which router is responsible for forwarding traffic from the source to the RP?
Rendezvous point (RP)
Why wrong: RP receives traffic from the first-hop router and sends it down the shared tree.
First-hop router
The source's DR unicasts the traffic to the RP.
Last-hop router
Why wrong: Last-hop router joins the shared tree for receivers.
Source-specific router
Why wrong: Not a standard PIM term.
Answer choices
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
First-hop router
In PIM sparse mode, the first-hop router (the router directly connected to the multicast source) is responsible for encapsulating the source's multicast traffic in unicast PIM register messages and forwarding them to the rendezvous point (RP). This process establishes the initial path and triggers the RP to join the source-specific tree (SPT) toward the source.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
Rendezvous point (RP)
Why it's wrong here
RP receives traffic from the first-hop router and sends it down the shared tree.
First-hop router
Why this is correct
The source's DR unicasts the traffic to the RP.
Related concept
Read the scenario before looking for a memorised answer.
Last-hop router
Why it's wrong here
Last-hop router joins the shared tree for receivers.
Source-specific router
Why it's wrong here
Not a standard PIM term.
Common exam traps
Cisco often tests the misconception that the RP originates or forwards traffic from the source, when in fact the first-hop router is the one that encapsulates and sends the source traffic to the RP using PIM register messages.
Detailed technical explanation
When the first-hop router receives multicast traffic from the source, it encapsulates each packet in a unicast PIM register message (destination IP = RP's address) and sends it to the RP. The RP decapsulates the traffic and forwards it down the shared tree (RP-tree) to receivers. Once the RP receives the first register message, it sends a PIM join (S,G) back toward the source, triggering the creation of an SPT between the source and the RP, after which the RP sends a register-stop message to the first-hop router to stop encapsulation.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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.
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FAQ
Network — This question tests Network — Read the scenario before looking for a memorised answer..
The correct answer is: First-hop router — In PIM sparse mode, the first-hop router (the router directly connected to the multicast source) is responsible for encapsulating the source's multicast traffic in unicast PIM register messages and forwarding them to the rendezvous point (RP). This process establishes the initial path and triggers the RP to join the source-specific tree (SPT) toward the source.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 24, 2026
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