The purpose of a dynamically created tunnel interface in the design of IPv6 multicast services is to transport all IPv6 multicast traffic. A tunnel interface is a virtual interface that is used to encapsulate IPv6 multicast traffic in an IPv4 or another IPv6 unicast packet for transport across a network. In the context of IPv6 multicast services, the tunnel interface is dynamically created when multicast traffic needs to be transported between multicast domains, or when the network topology requires it. The tunnel interface is used to encapsulate the IPv6 multicast traffic, allowing it to traverse the network as a unicast packet, which is then decapsulated and forwarded to the appropriate receivers.
I don't agree, there is indeed an Automated Multicast Tunnel feature to carry multicast over a unicast network, but this is not general IPv6 operation. I feel B is correct, the PIM register message from the first-hop router is carried in a tunnel
D - Question is asking about the Design of Ipv6 Multicast, not the operation of it (otherwise it would be A). You would consider it in a design for the reason outlined in D
According to 350-401 certification guide. Page 349. Chapter 13
In Figure 13-17, as soon as the source for a group G sends a packet, the FHR that is attached to this source is responsible for registering this source with the RP and requesting the RP to build a tree back to that router.
The FHR encapsulates the multicast data from the source in a special PIM-SM message called the register message and unicasts that data to the RP using a unidirectional PIM tunnel.
When the RP receives the register message, it decapsulates the multicast data packet inside the register message, and if there is no active shared tree because there are no interested receivers, the RP sends a register stop message directly to the registering FHR, without traversing the PIM tunnel, instructing it to stop sending the register messages.
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_pim/configuration/xe-16/imc-pim-xe-16-book/ip6-mcast-pim-sm.html
A multicast data sender sends data destined for a multicast group. The designated router (DR) of the sender takes those data packets, unicast-encapsulates them, and sends them directly to the RP. The RP receives these encapsulated data packets, de-encapsulates them, and forwards them onto the shared tree. The packets then follow the (*, G) multicast tree state in the devices on the RP tree, being replicated wherever the RP tree branches, and eventually reaching all the receivers for that multicast group. The process of encapsulating data packets to the RP is called registering, and the encapsulation packets are called PIM register packets.
and you see this:
Device# show ipv6 pim tunnel
Displays information about the PIM register encapsulation and de-encapsulation tunnels on an interface.
I think this should be A (First Hop Registration) - caused by PIM joins
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