The answer is that the interface must be configured with 'ip igmp version 3' to support Source-Specific Multicast (SSM). SSM requires IGMPv3 because it is the only version of IGMP that allows hosts to specify a particular source when joining a multicast group, enabling the router to build source-specific trees rather than shared trees. Without IGMPv3, the router defaults to IGMPv2, which only supports any-source multicast and cannot filter traffic from specific sources, breaking the core functionality of SSM. On the ENCOR 350-401 exam, this is a classic trap: candidates often remember to enable PIM sparse-mode but overlook that SSM is inherently tied to IGMPv3 on the last-hop router, and the SSM range 232.0.0.0/8 is automatically enabled once IGMPv3 is configured. A quick memory tip is "SSM needs three—IGMPv3," since the version number matches the initial of Source-Specific Multicast.
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.
Given the following configuration on a Cisco IOS-XE router:
ip multicast-routing
!
interface GigabitEthernet0/0
ip address 10.1.1.1 255.255.255.0
ip pim sparse-mode
ip igmp version 2
!
router ospf 1
network 10.1.1.0 0.0.0.255 area 0
!
What is missing from this configuration to support Source-Specific Multicast (SSM) for group range 232.0.0.0/8?
The interface must be configured with 'ip igmp version 3' to support SSM.
Correct. IGMPv3 is required for SSM to allow hosts to specify sources in their membership reports.
B
The router must be configured with 'ip pim ssm' command to enable SSM.
Why wrong: Incorrect. The 'ip pim ssm' command is optional; by default, the SSM range (232.0.0.0/8) is enabled when IGMPv3 is used. However, the command can be used to change the default range.
C
An RP must be configured for the 232.0.0.0/8 range.
Why wrong: Incorrect. SSM does not use an RP; it is source-specific.
D
The interface must be configured with 'ip pim dense-mode' for SSM to work.
Why wrong: Incorrect. SSM uses PIM sparse-mode (or PIM SSM) without an RP.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The interface must be configured with 'ip igmp version 3' to support SSM.
SSM requires IGMPv3 to allow hosts to specify sources. The configuration uses IGMPv2, which does not support source filtering. Additionally, PIM sparse-mode is used, but SSM typically uses PIM SSM (which is essentially PIM sparse-mode with the SSM range). The missing piece is IGMPv3; the SSM range is automatically enabled for 232.0.0.0/8 when IGMPv3 is configured.
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 must be configured with 'ip igmp version 3' to support SSM.
Why this is correct
Correct. IGMPv3 is required for SSM to allow hosts to specify sources in their membership reports.
The router must be configured with 'ip pim ssm' command to enable SSM.
Why it's wrong here
Incorrect. The 'ip pim ssm' command is optional; by default, the SSM range (232.0.0.0/8) is enabled when IGMPv3 is used. However, the command can be used to change the default range.
✗
An RP must be configured for the 232.0.0.0/8 range.
Why it's wrong here
Incorrect. SSM does not use an RP; it is source-specific.
✗
The interface must be configured with 'ip pim dense-mode' for SSM to work.
Why it's wrong here
Incorrect. SSM uses PIM sparse-mode (or PIM SSM) without an RP.
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. The 'ip pim ssm' command is optional; by default, the SSM range (232.0.0.0/8) is enabled when IGMPv3 is used. However, the command can be used to change the default range.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 350-401 question in full detail.
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.
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 must be configured with 'ip igmp version 3' to support SSM. — SSM requires IGMPv3 to allow hosts to specify sources. The configuration uses IGMPv2, which does not support source filtering. Additionally, PIM sparse-mode is used, but SSM typically uses PIM SSM (which is essentially PIM sparse-mode with the SSM range). The missing piece is IGMPv3; the SSM range is automatically enabled for 232.0.0.0/8 when IGMPv3 is configured.
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.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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