As we already know from the networking basics, the main job of the router is to route packets, that means when it receives an IP packet it will look to the layer 3 destination address then check its routing table to see what the next hop should be to forward the IP packet to and forward it from the right interface. In case he doesn’t see an entry in his routing table, then he will discard the packet.
Let me visualize this with the below illustration:
R2 has received a packet with a destination to 192.168.2.2, it will look to its routing table and there is an entry on his routing table saying that if you want to send anything to 192.168.2.0/24 then you have to forward the packet out of Ether2 interface. Then R2 will forward the packet to the destination devices.
All what you have seen up to now applied on unicast traffic. Are you wondering what would happen in case of multicast traffic? Let me show you another example.
Above you see that R1 has received a multicast traffic from a video server with a destination of 220.127.116.11 which is a multicast address. On his routing table, he has only the unicast destination, that means that this is a unicast routing table. If we leave the router like this, he will never know how to send the multicast packet because he has no idea where to forward the multicast traffic to. For this reason, we need to configure a multicast routing protocol and the only one that is available on MikroTik is PIM-SM (Protocol Independent Multicast – Sparse Mode) that most of the time we refer to it as just PIM.