What is the purpose of RP (Rendezvous Point) in multicast routing?

In multicast routing, the Rendezvous Point (RP) is a key component used in Protocol Independent Multicast (PIM) protocols, such as PIM-Sparse Mode (PIM-SM) and PIM-Source-Specific Multicast (PIM-SSM). The RP serves as a central point for managing and distributing multicast group information within a network.

The main purposes of the Rendezvous Point are:

1. Group Membership Management: The RP is responsible for keeping track of which multicast groups have active members within the network. When a host wants to join a particular multicast group, it registers with the RP to become a member of that group.
2. Distribution of Multicast Traffic: The RP facilitates the distribution of multicast traffic to the members of a multicast group. When a source starts sending multicast data for a specific group, the RP helps in determining the path for delivering the data to the group members.
3. RP Selection in PIM-SM: In PIM-SM, the Rendezvous Point is especially crucial. PIM-SM separates the distribution tree creation and maintenance from the source to receiver. The RP is the central point where sources register and receivers join, and the RP maintains the information about active sources and receivers for each multicast group.
4. Scalability: The use of an RP helps improve the scalability of multicast routing. Instead of having all routers in the network maintain information about all multicast groups, the RP centralizes the group membership information, reducing the amount of multicast-related state information that each router needs to handle.
5. Source Discovery: The RP is involved in the process of source discovery, helping routers and hosts discover the location of the source of a particular multicast group.

PIM-SM relies heavily on the concept of an RP, PIM-SSM, another PIM mode, does not use an RP. In PIM-SSM, multicast sources and receivers explicitly communicate with each other, eliminating the need for a centralized Rendezvous Point.