Multicast PIM Register Message

Hello Vadim

When a source router sends a registration request, it sends it to the RP that corresponds to the specific group. When there are multiple RPs, each multicast router maintains a group-to-RP mapping where the groups correspond to the appropriate RPs. Using the show ip pim rp mapping command, you can see these mappings. How are they created? It depends upon the setup.

You can use mechanisms such as Auto-RP or BSR to automatically choose an RP. These mechanisms also maintain this mapping that is used in the event that there are multiple RPs.

Multiple RPs can be statically configured as well. As seen from the Wireshark capture in the PIM_Register section of the Multicast Sparse Mode lesson, the registration request is sent specifically to the RP with an address of, and that RP is chosen from the group to RP mappings. Now in this case there is only one RP, but if there were more, the choice is made from the mappings.

I hope this has been helpful!