350-401 Question #1275: Real Exam Question with Answer & Explanation

Question

How does IGMP function with multicast routing and PIM?

Options

• AIGMP is enabled automatically when multicast routing and PIM are configured on a router
• BIGMP is incompatible with multicast routing
• CIGMP is incompatible with PIM
• DIGMP must be enabled manually when multicast routing and PIM are configured on a router

Explanation

IGMP with Multicast Routing and PIM

When multicast routing and PIM (Protocol Independent Multicast) are configured on a Cisco router, IGMP is automatically enabled on all active interfaces - no separate manual configuration is required. This makes sense by design, since IGMP is the foundational protocol that allows hosts to signal their interest in multicast group membership, and PIM cannot function meaningfully without knowing which groups have active receivers.

Options B and C are clearly incorrect because IGMP is not only compatible with both multicast routing and PIM - it is an essential component that works alongside them to build and maintain multicast distribution trees. Option D is the most tempting distractor, but it reverses reality: Cisco IOS automatically activates IGMP when you enable ip multicast-routing and ip pim on interfaces, so manual IGMP enablement is unnecessary.

🧠 Memory Tip: Think of IGMP as the "automatic welcome mat" - the moment you open the door with multicast routing and PIM, IGMP rolls out on its own to greet hosts wanting multicast traffic. Auto = A = Answer A!

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