300-410 Layer 3 Technologies Practice Question
A network engineer is configuring a GRE tunnel between two Cisco IOS-XE routers to transport multicast traffic. The engineer notices that multicast packets are not being forwarded through the tunnel. Which action should the engineer take to enable multicast over the GRE tunnel?
⚠ Common exam trap
The trap here is assuming that GRE tunnels automatically carry multicast traffic, when in fact multicast routing and PIM must be explicitly enabled on both the tunnel and physical interfaces.
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
✓
Enable IP multicast routing globally and configure PIM on the tunnel interface and the physical interface.
To transport multicast over a GRE tunnel, multicast routing must be enabled globally, and PIM must be configured on both the tunnel interface and the physical interface. This allows the router to forward multicast packets into the tunnel and out the physical interface. The other options either misconfigure PIM, use an unnecessary tunnel mode, or address an unrelated MTU issue.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Increase the tunnel MTU to accommodate multicast packets, as fragmentation prevents multicast forwarding.
Why it's wrong here
MTU issues can cause fragmentation and performance problems, but they do not prevent multicast forwarding entirely. Multicast packets would still be forwarded if fragmentation occurs, albeit less efficiently. The primary reason multicast fails over GRE is the lack of multicast routing and PIM configuration, not MTU size. Adjusting MTU would not enable multicast.
- ✗
Configure the tunnel mode to gre multipoint to support multicast.
Why it's wrong here
GRE multipoint (mGRE) is used for DMVPN and supports multicast only when combined with NHRP and multicast mapping. Simply changing the tunnel mode to gre multipoint does not automatically enable multicast forwarding. Moreover, the scenario describes a point-to-point GRE tunnel, so mGRE is not required. The correct step is to enable multicast routing and PIM.
- ✓
Enable IP multicast routing globally and configure PIM on the tunnel interface and the physical interface.
Why this is correct
GRE tunnels can carry multicast traffic if multicast routing is enabled. You must enable ip multicast-routing globally and configure PIM (e.g., PIM sparse-mode) on both the tunnel interface and the underlying physical interface. This allows multicast packets to be encapsulated and forwarded through the tunnel. Without PIM on the tunnel, multicast traffic will not be routed.
- ✗
Set the tunnel interface to ip pim sparse-dense-mode and enable ip multicast-routing on the physical interface only.
Why it's wrong here
Enabling PIM on the tunnel interface is necessary, but you also need to enable multicast routing globally and configure PIM on the physical interface that carries the tunnel. Configuring PIM only on the physical interface is insufficient because the tunnel interface itself must participate in multicast routing. The command ip pim sparse-dense-mode is valid, but the placement is incomplete.
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JA
Written and reviewed by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
Last reviewed September 2026 · checked against the official Cisco exam blueprint
This 300-410 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 300-410 exam.