Question 1,388 of 2,015

IP Multicast →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that PIM sparse-mode requires an RP to be configured or learned for the protocol to function correctly. This is because PIM sparse-mode builds multicast distribution trees only on demand, using a rendezvous point (RP) as the central meeting point for senders and receivers; without an RP, the router cannot join or forward multicast traffic in sparse mode. On the ENCOR 350-401 exam, this configuration tests your understanding that enabling `ip pim sparse-mode` on an interface is necessary but insufficient—you must also statically define an RP or use a dynamic mechanism like Auto-RP or BSR. A common trap is assuming IGMPv3 alone enables Source-Specific Multicast (SSM) for all groups, but SSM only works within the 232.0.0.0/8 range; outside that range, sparse mode still needs an RP. Memory tip: think of PIM sparse-mode as a VIP party—you need the RP as the bouncer (the meeting point) or no one gets in.

350-401 IP Multicast Practice Question

This 350-401 practice question tests your understanding of ip multicast. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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.

Consider the following configuration snippet on a Cisco IOS-XE router:

interface GigabitEthernet0/1
 ip address 10.1.1.1 255.255.255.0
 ip pim sparse-mode
 ip igmp version 3

router ospf 1
 network 10.1.1.0 0.0.0.255 area 0

What is the effect of this configuration?

Question 1mediummultiple choice

Review the full OSPF breakdown →

A
The interface will participate in PIM sparse-mode and IGMPv3, but PIM sparse-mode requires an RP to be configured or learned.
Correct. PIM sparse-mode requires an RP. IGMPv3 is supported but the RP must be defined for sparse-mode to work.
B
The interface will operate in PIM dense-mode because no RP is configured.
Why wrong: Incorrect. PIM sparse-mode is explicitly configured; without an RP, the router will not forward multicast traffic correctly, but it does not fall back to dense-mode.
C
IGMPv3 is incompatible with PIM sparse-mode and will be ignored.
Why wrong: Incorrect. IGMPv3 works with PIM sparse-mode and is actually required for SSM.
D
The router will automatically use dense-mode because OSPF is enabled.
Why wrong: Incorrect. OSPF is a unicast routing protocol and does not affect PIM mode selection.

Full breakdown with real-world context →

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 interface will participate in PIM sparse-mode and IGMPv3, but PIM sparse-mode requires an RP to be configured or learned.

The interface is configured with PIM sparse-mode and IGMPv3. PIM sparse-mode requires a rendezvous point (RP) to be known, either statically or via Auto-RP/BSR. IGMPv3 allows source-specific multicast (SSM) with the SSM range (232.0.0.0/8). OSPF is enabled on the interface for unicast routing. The configuration is valid but missing an RP definition for sparse-mode to function correctly.

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 interface will participate in PIM sparse-mode and IGMPv3, but PIM sparse-mode requires an RP to be configured or learned.
Why this is correct
Correct. PIM sparse-mode requires an RP. IGMPv3 is supported but the RP must be defined for sparse-mode to work.
Related concept
OSPF neighbours must agree on key parameters.
✗
The interface will operate in PIM dense-mode because no RP is configured.
Why it's wrong here
Incorrect. PIM sparse-mode is explicitly configured; without an RP, the router will not forward multicast traffic correctly, but it does not fall back to dense-mode.
✗
IGMPv3 is incompatible with PIM sparse-mode and will be ignored.
Why it's wrong here
Incorrect. IGMPv3 works with PIM sparse-mode and is actually required for SSM.
✗
The router will automatically use dense-mode because OSPF is enabled.
Why it's wrong here
Incorrect. OSPF is a unicast routing protocol and does not affect PIM mode selection.

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 350-401 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 interface will participate in PIM sparse-mode and IGMPv3, but PIM sparse-mode requires an RP to be configured or learned. — The interface is configured with PIM sparse-mode and IGMPv3. PIM sparse-mode requires a rendezvous point (RP) to be known, either statically or via Auto-RP/BSR. IGMPv3 allows source-specific multicast (SSM) with the SSM range (232.0.0.0/8). OSPF is enabled on the interface for unicast routing. The configuration is valid but missing an RP definition for sparse-mode to function correctly.

What should I do if I get this 350-401 question wrong?

What is the key concept behind this question?

OSPF neighbours must agree on key parameters.

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