By default all sources are allowed to register at the RP (Rendezvous Point) when using PIM sparse mode. When a new source starts sending traffic to a multicast group address, the PIM DR (Designated Router) on the segment connected to the source will forward a PIM register message to the RP.
This PIM register message contains the original multicast packet from the source, and it includes the IP address of the source and the destination multicast group address. We can configure our RP to filter certain sources, if we do this the RP will send a PIM register-stop message to the PIM DR so that it will not build the SPT towards the source.
Let’s take a look at an example topology:
Above we have two routers, R1 and R2. We will configure R1 as a rendezvous point and R2 will be our source. Let’s configure the basics first:
R1(config)#ip multicast-routing R1(config)#ip pim rp-address 192.168.12.1 R1(config)#interface fastEthernet 0/0 R1(config-if)#ip pim sparse-mode
R2(config)#ip multicast-routing R2(config)#ip pim rp-address 192.168.12.1 R2(config)#interface fastEthernet 0/0 R2(config-if)#ip pim sparse-mode
Just a simple PIM sparse mode setup where R1 is the RP. By default all sources and all multicast groups are allowed to register at the RP.
Let’s send some traffic to a multicast group address from R2 to see what R1 thinks of it:
R1#debug ip pim PIM debugging is on
First we will enable a debug on R1, now let’s send some traffic from R2:
R2#ping 126.96.36.199 repeat 9999 Type escape sequence to abort. Sending 9999, 100-byte ICMP Echos to 188.8.131.52, timeout is 2 seconds: ...
This is what you will see on R1:
R1# PIM(0): Received v2 Register on FastEthernet0/0 from 192.168.12.2
It receives a PIM register message from 192.168.12.2. Nobody is listening to this multicast stream but you will find it in the multicast routing table: