Exam guide: "In essence, two trees are created: an SPT from the FHR to the RP (S,G) and a shared tree from the RP to the LHR (*,G)". The RP is the root of the shared tree in PIM sparse mode, C is correct.
Answer is C.
Multicast Distribution Shared Tree - Unlike source trees that have their root at the source, shared trees use a single common root placed at some chosen point in the network. This shared root is called a rendezvous point (RP).
Source: https://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst9300/software/release/16-5/configuration_guide/ip_mcast_rtng/b_165_ip_mcast_rtng_9300_cg/b_165_ip_mcast_rtng_9300_9500_cg_chapter_0100.html
When hosts join a multicast group, the directly connected routers send PIM Join messages toward the RP. The RP keeps track of multicast groups. Hosts that send multicast packets are registered with the RP by the first hop router of that host. The RP then sends Join messages toward the source. At this point, packets are forwarded on a shared distribution tree.
My English i not good enough to understand tricky nuances of Cisco questions, but IMHO:
The RP will not responds to the PIM join messages with "THE SOURCE" of a requested multicast group, but rather will start forwarding multicast traffic in direction of requester.
So "C" would be a better answer.
D. The RP responds to the PIM join messages with the source of a requested multicast group.
In PIM (Protocol Independent Multicast) Sparse Mode, the RP (Rendezvous Point) plays a crucial role in facilitating the multicast traffic delivery to the receivers. The RP is responsible for keeping track of the multicast groups and their associated sources in the network. It receives the join messages from the routers and forwards them to the appropriate source(s) of the group. The RP also maintains a mapping between the multicast group address and the source(s) associated with it. When the RP receives the multicast traffic from the source(s), it forwards it to all the routers in the network that have requested to join the group.
Answer is 100% C;
D is incorrect because RP does not respond to a Join message, it simply forwards the multicast stream to the reciever . ( See also answer from "RTE" and "lukaszr")
Rather than guessing based on 3 sentence long paragraph, I encourage you to spend 5 min to read RFC 4609, section 3.3.1.
RP doesn't respond to any join messages.
Correct answer is C.
may be D better
In sparse mode, a router assumes that other routers do not want to forward multicast packets for a group, unless there is an explicit request for the traffic. When hosts join a multicast group, the directly connected routers send PIM Join messages toward the RP. The RP keeps track of multicast groups. Hosts that send multicast packets are registered with the RP by the first hop router of that host. The RP then sends Join messages toward the source. At this point, packets are forwarded on a shared distribution tree. If the multicast traffic from a specific source is sufficient, the first hop router of the host may send Join messages toward the source to build a source-based distribution tree.
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_pim/configuration/xe-16-5/imc-pim-xe-16-5-book/imc-tech-oview.html#GUID-8168D184-0F45-4EAA-B9C0-68403809DE77
Where do you see in the quoted paragraph any info about RESPONDING to any Join messages? RP doesn't respond to join message. It sends its own Join messages towards sources.
C is the only correct answer.
C
From Guide: "Figure 13-17 illustrates a multicast source sending multicast traffic to the FHR. The FHR
then sends this multicast traffic to the RP, which makes the multicast source known to the
RP. It also illustrates a receiver sending an IGMP join to the LHR to join the multicast group.
The LHR then sends a PIM join (*,G) to the RP, and this forms a shared tree from the RP to
the LHR. The RP then sends a PIM join (S,G) to the FHR, forming a source tree between the source and the RP. <!>In essence, two trees are created: an SPT from the FHR to the RP (S,G) and a shared tree from the RP to the LHR (*,G)." <!>
Sparse Mode
Sparse mode operation centers around a single unidirectional shared tree whose root node is called the rendezvous point (RP). Sources must register with the RP to get their multicast traffic to flow down the shared tree by way of the RP. This registration process actually triggers a shortest path tree (SPT) Join by the RP toward the source when there are active receivers for the group in the network.
A sparse mode group uses the explicit join model of interaction. Receiver hosts join a group at a rendezvous point (RP). Different groups can have different RPs.
Multicast traffic packets flow down the shared tree to only those receivers that have explicitly asked to receive the traffic.
https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipmulti_pim/configuration/xe-16-5/imc-pim-xe-16-5-book/imc-tech-oview.html
An RP acts as the meeting place for sources and receivers of multicast data. In a PIM-SM network, sources must send their traffic to the RP. This traffic is then forwarded to receivers down a shared distribution tree.
C is correct.
In PIM-SM, source sends traffic to RP then fwd it towards the reciver. when the receiving router gets the traffic, it will send join message directly to the source creating a source-based distribution tree from the source to the receiver. This mesg does not include RP that's why RP is only needed at the start of the session. C is correct and RP is the root of out tree.
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