48 MSDP C ONFIGURATIONMSDP OverviewIntroduction to MSDP Multicast Source Discovery Protocol (MSDP) is an inter-domain multicast solutiondeveloped to address the interconnection of Protocol Independent Multicastsparse mode (PIM-SM) domains. It is used to discover multicast source informationin other PIM-SM domains.In the basic PIM-SM mode, a multicast source registers only with the RP in the localPIM-SM domain, and the multicast source information of a domain is isolatedfrom that of another domain. As a result, the RP is aware of the sourceinformation only within the local domain and a multicast distribution tree is builtonly within the local domain to deliver multicast data from a local multicast sourceto local receivers. If there is a mechanism that allows RPs of different PIM-SMdomains to share their multicast source information, the local RP will be able tojoin multicast sources in other domains and multicast data can be transmittedamong different domains.MSDP achieves this objective. By establishing MSDP peer relationships among RPsof different PIM-SM domains, source active (SA) messages can be forwardedamong domains and the multicast source information can be shared.c CAUTION:■ MSDP is applicable only if the intra-domain multicast protocol is PIM-SM.■ MSDP is meaningful only for the any-source multicast (ASM) model.How MSDP Works MSDP peersWith one or more pairs of MSDP peers configured in the network, an MSDPinterconnection map is formed, where the RPs of different PIM-SM domains areinterconnected in series. Relayed by these MSDP peers, an SA message sent by anRP can be delivered to all other RPs.
