Inter-domain Multicast Source Information Transmission Among Domains

As shown in Figure 6-4, the PIM-SM network is divided into four PIM-SM domains. There is an active multicast source (Source) in the domain PIM-SM1, and RP1 knows the existence of this source after the source registers to RP1. Domains PIM-SM2 and PIM-SM3 want to know the exact position of this source to receive multicast data from it. MSDP peer relationships must be set up between RP1 and RP2, and between RP2 and RP3.

Figure 6-4 Inter-domain multicast source information transmission

Multicast source information is transmitted among domains under the following process:

1. When Source in PIM-SM1 sends the first multicast packet to the multicast group, the designated router DR1 encapsulates multicast data to a register message and sends the message to RP1. RP1 then obtains information about this multicast source.
2. As a source RP, RP1 creates SA messages and periodically sends SA messages to its peer RP2. SA messages contain the multicast source address S, the group address G, and the address of the source RP1 that creates the SA message.
3. After receiving SA messages, RP2 performs a reverse path forwarding (RPF) check. RP2 forwards messages that pass the RPF check to RP3, and checks whether there is a member of group G in the local domain. Because PIM-SM2 contains no receiver of group G, RP2 does not perform any other action.
4. After RP3 receives the SA message, it performs an RPF check on the message. The check succeeds. Because a member of group G locates in PIM-SM3, RP3 generates a (*, G) entry using IGMP.
5. RP3 creates an (S, G) entry and sends a Join message with (S, G) information to Source hop by hop. A multicast path (the shortest path tree SPT) from Source to RP3 is then set up. After multicast data reaches RP3 along the SPT, RP3 forwards the data to the receiver along the RPT.
6. After the receiver DR3 receives multicast data from Source, it determines whether to initiate an SPT switchover.