1-71) The multicast source registers with the nearest RP. In this example, Source registers with RP 1,with its multicast data encapsulated in the register message. When the register message arrives atRP 1, RP 1 decapsulates the message.2) Receivers send join messages to the nearest RP to join in the RPT rooted as this RP. In thisexample, Receiver joins the RPT rooted at RP 2.3) RPs share the registered multicast information by means of SA messages. In this example, RP 1creates an SA message and sends it to RP 2, with the multicast data from Source encapsulated inthe SA message. When the SA message reaches RP 2, RP 2 decapsulates the message.4) Receivers receive the multicast data along the RPT and directly join the SPT rooted at the multicastsource. In this example, RP 2 forwards the multicast data down the RPT. When Receiver receivesthe multicast data from Source, it directly joins the SPT rooted at Source.The significance of Anycast RP is as follows:z Optimal RP path: A multicast source registers with the nearest RP so that an SPT with the optimalpath is built; a receiver joins the nearest RP so that an RPT with the optimal path is built.z Load balancing between RPs: Each RP just needs to maintain part of the source/group informationwithin the PIM-SM domain and forward part of the multicast data, thus achieving load balancingbetween different RPs.z Redundancy backup between RPs: When an RP fails, the multicast source previously registeredon it or the receivers previous joined it will register with or join another nearest RP, thus achievingredundancy backup between RPs.z Be sure to configure a 32-bit subnet mask (255.255.255.255) for the Anycast RP address, namelyconfigure the Anycast RP address into a host address.z An MSDP peer address must be different from the Anycast RP address.Multi-Instance MSDPMSDP peering relationship can be built between multicast-enabled interfaces that belong to the sameinstance. Through exchanges of SA messages between MSDP peers, the MSDP mechanism makesVPN multicast transmission between different PIM-SM domains possible.A multicast router running multiple MSDP instances maintains an independent set of MSDP mechanismfor each instance it supports, including SA cache, peering connection, timers, sending cache, andcache for exchanging information with PIM, while one instance is isolated from another; therefore,interoperability between MSDP and PIM-SM is available only within the same instance.Protocols and StandardsMSDP is documented in the following specifications:z RFC 3618: Multicast Source Discovery Protocol (MSDP)z RFC 3446: Anycast Rendezvous Point (RP) mechanism using Protocol Independent Multicast(PIM) and Multicast Source Discovery Protocol (MSDP)
