- A
The RP address is not reachable via the unicast routing table.
Why wrong: Incorrect because the scenario states the RP is reachable via OSPF.
- B
The command 'ip pim rp-address 10.1.1.1' must include an access-list to define the group range.
Correct because the RP mapping requires an access-list to specify the groups; without it, the RP is not associated with any group.
- C
PIM sparse mode must be enabled on all interfaces first.
Why wrong: Incorrect because PIM can be enabled per interface, but the RP mapping command is global and does not require all interfaces to have PIM enabled.
- D
The router must be configured as a candidate RP using 'ip pim send-rp-announce'.
Why wrong: Incorrect because that command is used for Auto-RP or BSR, not for static RP configuration.
Quick Answer
The answer is that the `ip pim rp-address 10.1.1.1` command must include an access-list to define the group range. Without an access-list, the router has no way of knowing which multicast groups this static RP should serve, so the RP mapping is never installed in the PIM database—even though the RP itself is reachable via OSPF. This is a common trap on the ENCOR 350-401 exam: candidates often assume that simply specifying the RP address is enough, but the command syntax explicitly requires an access-list to bind the RP to specific group prefixes. The `show ip pim rp mapping` output will remain empty until that access-list is applied, which directly explains why multicast traffic fails to forward. Remember the memory tip: "No ACL, no RP mapping"—the access-list is the glue that connects the static RP address to the multicast groups it is meant to support.
CCNP IP Multicast Practice Question
This 350-401 practice question tests your understanding of ip multicast. 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.
An engineer is configuring multicast on a Cisco router running IOS-XE. The network uses PIM sparse mode with a static RP at 10.1.1.1. The engineer enters the command 'ip pim rp-address 10.1.1.1' but multicast traffic is not being forwarded. Upon verification, the engineer sees that the RP is reachable via OSPF, but the 'show ip pim rp mapping' command does not list any RP for the group. What is the most likely cause?
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.
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 command 'ip pim rp-address 10.1.1.1' must include an access-list to define the group range.
The 'ip pim rp-address' command requires an access-list to specify which groups the RP serves. Without the access-list, the command is incomplete and the RP mapping is not applied, even though the RP is reachable.
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.
- ✗
The RP address is not reachable via the unicast routing table.
Why it's wrong here
Incorrect because the scenario states the RP is reachable via OSPF.
- ✓
The command 'ip pim rp-address 10.1.1.1' must include an access-list to define the group range.
Why this is correct
Correct because the RP mapping requires an access-list to specify the groups; without it, the RP is not associated with any group.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
PIM sparse mode must be enabled on all interfaces first.
Why it's wrong here
Incorrect because PIM can be enabled per interface, but the RP mapping command is global and does not require all interfaces to have PIM enabled.
- ✗
The router must be configured as a candidate RP using 'ip pim send-rp-announce'.
Why it's wrong here
Incorrect because that command is used for Auto-RP or BSR, not for static RP configuration.
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.
Trap categories for this question
Command / output trap
Incorrect because PIM can be enabled per interface, but the RP mapping command is global and does not require all interfaces to have PIM enabled.
Scenario analysis trap
Incorrect because the scenario states the RP is reachable via OSPF.
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 350-401 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 350-401 question test?
IP Multicast — This question tests IP Multicast — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The command 'ip pim rp-address 10.1.1.1' must include an access-list to define the group range. — The 'ip pim rp-address' command requires an access-list to specify which groups the RP serves. Without the access-list, the command is incomplete and the RP mapping is not applied, even though the RP is reachable.
What should I do if I get this 350-401 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 350-401 OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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