Multicast PIM Auto RP

Lessons Discussion
86

Hi,

I noticed that when a router is configured as a Mapping Agent all of its interfaces join 224.0.1.39, whilst for an candidate RP, it’s just one (which makes sense). Why does it need to join on all interfaces?

Thanks.

Sam

87

Hello Sam

The Mapping Agent in PIM Auto-RP must join multicast group 224.0.1.39 on all interfaces to ensure it receives RP announce messages from any candidate RP in the network. It doesn’t know which routers may possibly be candidate RPs, so it has to be prepared to receive such messages on any interface. These announcements are multicast to 224.0.1.39 and can originate from any router configured as a candidate RP.

In contrast, a candidate RP only needs to send announcements to 224.0.1.39 and doesn’t require active listening on all interfaces. Its role is to send rather than to receive. If a router serves as both Candidate RP and Mapping Agent, it would join on all interfaces due to its Mapping Agent role. Does that make sense?

I hope this has been helpful!

Laz