620 BigIron RX Series Configuration Guide53-1001986-01Configuring Multicast Source Discovery Protocol (MSDP)23Peer Reverse Path Forwarding (RPF) floodingWhen the MSDP router (also the RP) in domain 2 receives the Source Active message from its peerin domain 1, the MSDP router in domain 2 forwards the message to all its other peers. Thepropagation process is sometimes called “peer Reverse Path Forwarding (RPF) flooding”. This termrefers to the fact that the MSDP router uses its PIM Sparse RPF tree to send the message to itspeers within the tree. In Figure 93, the MSDP router floods the Source Active message it receivesfrom its peer in domain 1 to its other peers, in domains 3 and 4.Note that the MSDP router in domain 2 does not forward the Source Active back to its peer indomain 1, because that is the peer from which the router received the message. An MSDP routernever sends a Source Active message back to the peer that sent it. The peer that sent the messageis sometimes called the “RPF peer”. The MSDP router uses the unicast routing table for its ExteriorGateway Protocol (EGP) to identify the RPF peer by looking for the route entry that is the next hoptoward the source. Often, the EGP protocol is Border Gateway Protocol (BGP) version 4.NOTEMSDP depends on BGP and MBGP for interdomain operations.The MSDP routers in domains 3 and 4 also forward the Source Active message to all their peersexcept the ones that sent them the message. Figure 93 does not show additional peers.Source active cachingWhen an MSDP router that is also an RP receives a Source Active message, the RP checks its PIMSparse multicast group table for receivers for the group. If the DR has a receiver for the group beingadvertised in the Source Active message, the DR sends a Join message for that receiver back tothe DR in the domain from which the Source Active message came. Usually, the DR is also theMSDP router that sent the Source Active message.In Figure 93, if the MSDP router and RP in domain 4 has a table entry for the receiver, the RP sendsa Join message on behalf of the receiver back through the RPF tree to the RP for the source, in thiscase the RP in domain 1.Some MSDP routers that are also RPs can cache Source Active messages. If the RP is not cachingSource Active messages, the RP does not send a Join message unless it already has a receiver thatwants to join the group. Otherwise, the RP does not send a Join message and does not rememberthe information in the Source Active message after forwarding it. If the RP receives a request froma receiver for the group, the RP and receiver must wait for the next Source Active message for thatgroup before the RP can send a Join message for the receiver.However, if Source Active caching is enabled on the MSDP and RP router, the RP caches the SourceActive messages it receives. In this case, even if the RP does not have a receiver for a group whenthe RP receives the Source Active message for the group, the RP can immediately send a Join for anew receiver that wants to join the group, without waiting for the next Source Active message fromthe RP in the source’s domain.The size of the cache used to store MSDP Source Active messages is 8KConfiguring MSDPTo configure MSDP on a BigIron RX, perform the following tasks:• Enable MSDP
