mediumMultiple Choice
350-401 Practice Question: Consider the following configuration snippet on a…
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?
⚠ Common exam trap
Cisco often tests the misconception that PIM sparse-mode automatically reverts to dense-mode when no RP is configured, but in reality, sparse-mode requires an explicit RP and will not forward traffic without one.
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 configuration explicitly enables PIM sparse-mode on the interface, which requires a rendezvous point (RP) to be known—either statically configured or dynamically learned via Auto-RP or BSR. The `ip igmp version 3` command enables IGMPv3, which is fully compatible with PIM sparse-mode and allows for source-specific multicast (SSM) support. OSPF is only used for unicast routing and does not affect PIM mode selection.
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
PIM sparse-mode is explicitly enabled on the interface with the ip pim sparse-mode command, which also activates IGMPv3 as the host membership protocol. In sparse mode, routers must send explicit join messages toward a rendezvous point (RP) to build multicast distribution trees. Without a configured or dynamically learned RP (e.g., via Auto-RP or BSR), the router cannot construct these shared trees, and multicast forwarding will fail. Therefore, the interface does participate in both PIM sparse-mode and IGMPv3, but the missing RP is a critical operational requirement.
- ✗
The interface will operate in PIM dense-mode because no RP is configured.
Why it's wrong here
This is incorrect because PIM does not automatically fall back to dense-mode when no RP is configured. Dense-mode is a separate mode that must be explicitly configured using ip pim dense-mode; it uses flood-and-prune behavior, which is fundamentally different from sparse-mode's explicit join mechanism. If a sparse-mode router lacks an RP, it simply cannot build multicast forwarding state and will not flood traffic. The absence of an RP causes an error condition (e.g., 'no RP configured') rather than a silent switch to dense-mode.
- ✗
IGMPv3 is incompatible with PIM sparse-mode and will be ignored.
Why it's wrong here
IGMPv3 is not incompatible with PIM sparse-mode; in fact, they work together and are complementary. IGMPv3 operates on the host-to-router segment, allowing hosts to specify source addresses in their membership reports, which is essential for Source-Specific Multicast (SSM). PIM sparse-mode is the protocol used between routers to build multicast trees, and it relies on IGMP to learn which groups are needed on a subnet. Far from being ignored, IGMPv3 is the default version on many Cisco interfaces and is a requirement for SSM services.
- ✗
The router will automatically use dense-mode because OSPF is enabled.
Why it's wrong here
OSPF is a unicast routing protocol that populates the routing table, and while that routing table is used for reverse-path forwarding (RPF) checks in multicast, it has no bearing on PIM mode selection. The PIM mode on an interface is determined solely by the ip pim sparse-mode or ip pim dense-mode configuration command. Enabling OSPF does not change the interface's PIM mode, nor does it cause the router to automatically use dense-mode. The two protocols operate at different layers of the multicast routing architecture: OSPF provides reachability, while PIM manages the distribution tree type.
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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