The RP is a central point that sees the communication between the source and the receiver. The receivers send IGMP Join messages to the first hop Layer 3 device, called the designated router. The designated router will send the packet to the RP. The RP should be configured on the Layer 3 device and also be reachable from the Layer 3 device through a PIM enabled interface. The RP will maintain the state for the respective groups.
Refer to PIM Sparse mode to understand the functionality of the RP in a multicast tree. This RP information in a network should be advertised to the other Layer 3 routers, referred to as downstream routers. The RP information can be advertised to the entire downstream Layer 3 devices through different methodologies:
• Static RP: All the downstream routers will have RP configuration enabled in the router.
• Auto-RP: The RP information is not configured in the downstream routers. This information is sent from the central RP distribution agent. The groups that function in Auto-RP (in PIM Dense mode) are 188.8.131.52 and 184.108.40.206. These two groups are needed for the propagation of RP information. Auto-RP supports Active/Standby RP redundancy. This is Cisco propriety.
• BSR: The Bootstrap Router (BSR) mechanism is available in PIM version 2. The RP information is not configured in the downstream routers. Devices are configured as candidate BSRs by enabling the ip pim bsr-candidate command. These devices announce themselves to other routers using the 220.127.116.11 group address. Because a message sent to this address is forwarded hop-by-hop throughout the network, all the routers learn about the candidate BSRs and select one of them as an RP. The RP is selected based on the highest priority in the BSR configuration. BSR supports Active/ Standby RP redundancy.
• Anycast RP: Anycast RP is an implementation strategy that provides load sharing and redundancy in PIM Sparse mode networks. Multicast Source Discovery Protocol (MSDP) is the key protocol, which makes the Anycast RP redundancy possible. Using Anycast RP, Active/Active RP redundancy is achieved. In Active/Active RP redundancy, both the RPs can function as redundant RPs for sources and receivers at the same time.
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