In the design ofIPv6 multicast services, particularly usingPIM Sparse Mode, thefirst-hop router(which is connected to the multicast source) must register the multicast source with theRendezvous Point (RP). This registration involves encapsulating the multicast packets in unicast messages and sending them to the RP.

This encapsulation is done over adynamically created tunnel interface, typically referred to as aPIM Register Tunnel. This temporary tunnel is established by the source-side router to send encapsulated multicast traffic to the RP for initial source registration.

Therefore, the dynamic tunnel serves the purpose of:

Encapsulating and sending the first multicast packets from the source to the RP

Allowing the RP to learn about new sources and initiate shared tree or shortest-path tree forwarding

This operation aligns with CCDE v3.1 design principles, where understanding the internal operation of multicast protocols—especially source registration and RP role—is critical for scalable and reliable multicast service delivery.

Why other options are incorrect:

A: First-hop router doesn’t register itself, it registers thesourceto the RP.

B: Clients (receivers) do not register to the RP; they send IGMP/MLD joins which flow toward the RP.

D: Not all IPv6 multicast traffic is transported via tunnels; only initial registration traffic is encapsulated in this manner.

This mechanism is essential for source discovery in sparse-mode multicast and is foundational to multicast tree building in IPv6 networks.