IPv4 multicast routing is the process of forwarding data packets from the source to one or more receivers residing in same or different networks. The source address of the multicast packet will be a unicast address, while the destination address will be a multicast address (224.0.0.0 to 239.255.255.255). The end applications that are using multicast to receive the traffic will resolve the multicast address using an out-of-band mechanism and will use a multicast group membership protocol like IGMP to join the respective multicast group. The host will send IGMP join towards the connected router.

The multicast-enabled router connecting the receivers is known as the Last Hop Router (LHR), and the multicast-enabled router connecting the source is known as the First Hop Router (FHR). The LHR will use a multicast routing protocol such as PIM to build a multicast tree toward the FHR using the shortest path. The FHR will forward multicast data traffic over the multicast tree. Multicast can be deployed in different modes. Below are the two most commonly deployed multicast modes:

• Sparse mode: In this mode, a common node will be positioned as a Rendezvous Point (RP) and each LHR will build the multicast tree towards the RP. This tree is known as a shared tree. The FHR, on receiving multicast traffic from a connected source, will unicast the packet to the RP, which in turn will forward it to the receivers over at the shared tree.
• Source-specific multicast: In this mode, each LHR will build the multicast tree towards the FHR connected to the source itself. There is no need for an RP in this mode.