- A
Increase the PIM register suppression interval on the source's first-hop router
Why wrong: This would only delay re-registration, not solve the root cause of the register not reaching the RP.
- B
Configure a static RP at the customer site (PE2) to bypass the firewall for registration traffic
A static RP on PE2 ensures that the source's registration reaches the RP even if the firewall blocks unicast PIM register messages. This allows the (S,G) to be formed.
- C
Change the multicast mode from PIM-SM to PIM-DM on all interfaces
Why wrong: PIM-DM uses flood-and-prune and would not solve the RP registration issue; it would also be inefficient for a sparse group.
- D
Enable Auto-RP on the network to dynamically learn the RP
Why wrong: Auto-RP uses multicast advertisements that may also be blocked by the firewall, and it does not address the immediate issue of lost registrations.
Quick Answer
The answer is to configure a static RP at the customer site (PE2) to bypass the firewall for registration traffic. This is correct because the firewall upgrade likely blocked PIM register messages from the source-connected PE1 to the static RP at 10.1.1.1, preventing the RP from learning the source and building the (S,G) entry; without that registration process, the (*,G) entry remains but the source-specific state never forms. On the Cisco SPCOR 350-501 exam, this scenario tests your understanding of PIM-SM’s reliance on unicast register messages and how a firewall can silently drop them even when multicast data flows, a common trap where engineers focus on multicast data paths rather than control-plane signaling. Remember the key troubleshooting step: if you see (*,G) but no (S,G) in PIM-SM, the RP never received the source’s registration—so check firewall rules for PIM register traffic (UDP port 16384) or place the RP where it can receive those messages. Memory tip: “No S,G? Check the register—firewalls love to block the unicast messenger.”
350-501 Networking Practice Question
This 350-501 practice question tests your understanding of networking. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 service provider is experiencing intermittent multicast issues in their core network. They use PIM-SM with a static RP at 10.1.1.1. The multicast traffic originates from a source connected to PE1 and is received by customers connected to PE2. Recently, after a firewall upgrade between the PE routers and the core, some multicast streams stopped working, while others continue. The network team notices that 'show ip mroute' on PE2 shows the (*, G) entry but not the (S, G) entry for the affected groups. The RP is reachable via OSPF. The firewall logs show no dropped packets for known multicast addresses. Which action should the engineer take to restore full multicast forwarding?
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
Configure a static RP at the customer site (PE2) to bypass the firewall for registration traffic
Option B is correct. The firewall upgrade likely blocked PIM register messages from PE1 to the RP, preventing the RP from learning about the source. By configuring a static RP on the customer-facing interface or using a different RP that can receive registration, the (S,G) state can be built. Option A is wrong because PIM-SM is correct for sparse-mode groups. Option C is wrong because adjusting timers would not fix the absence of (S,G). Option D is wrong because Auto-RP would add complexity and might be blocked by the firewall as well.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the PIM register suppression interval on the source's first-hop router
Why it's wrong here
This would only delay re-registration, not solve the root cause of the register not reaching the RP.
- ✓
Configure a static RP at the customer site (PE2) to bypass the firewall for registration traffic
- ✗
Change the multicast mode from PIM-SM to PIM-DM on all interfaces
Why it's wrong here
PIM-DM uses flood-and-prune and would not solve the RP registration issue; it would also be inefficient for a sparse group.
- ✗
Enable Auto-RP on the network to dynamically learn the RP
Why it's wrong here
Auto-RP uses multicast advertisements that may also be blocked by the firewall, and it does not address the immediate issue of lost registrations.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
What to study next
Got this wrong? Here's your next step.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 350-501 question test?
Networking — This question tests Networking — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Configure a static RP at the customer site (PE2) to bypass the firewall for registration traffic — Option B is correct. The firewall upgrade likely blocked PIM register messages from PE1 to the RP, preventing the RP from learning about the source. By configuring a static RP on the customer-facing interface or using a different RP that can receive registration, the (S,G) state can be built. Option A is wrong because PIM-SM is correct for sparse-mode groups. Option C is wrong because adjusting timers would not fix the absence of (S,G). Option D is wrong because Auto-RP would add complexity and might be blocked by the firewall as well.
What should I do if I get this 350-501 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 350-501 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 24, 2026
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