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If you want to send a message from one source to one destination, you could send a unicast message. If you want send a message from one source to all the destination on local network, you could send a broadcast message. However, if you want to send a message from on source to selected multiple destinations spread across a routed network in one data stream, the most efficient method is IP multicasting.



Multicast routing differs from unicast routing in several ways. The most important differences are in the ways that multicast routers use source and destination addresses. A multicast packet is addressed to a special IP address representing a group of devices that can be scattered anywhere throughout a network. Since the destinations can be anywhere, the only reliable way to eliminate loops in multicast routing is to look at the reverse path back to the source. So, while unicast routing cares where the packet is going, multicast routing also needs to know where it came from.


For this reason, multicast routing protocols such as Protocol Independent Multicast (PIM) always work with the source address and destination group simultaneously. The usual notation for a multicast route is (Source, Group), as opposed to the unicast case in which routes are defined by the destination address alone. We have already mentioned that this is necessary for avoiding loops, but the router also needs to keep track of both source and group addresses in each multicast routing table entry because there could be several sources for the same group.


When you look at the multicast routing table with the show ip mroute command, you will see not only (Source, Group) pairs like (,, but also pairs that look like (*, This means that the source is unspecified. Cisco routers organize their multicast routing tables with a parent (*, Group) for each group, and any number of (Source, Group) pairs under it. If there is a (*, Group) but no (Source, Group) entries for a group, then that just means that the router knows of group members but doesn't yet know where to expect this multicast traffic from.


Each of these (Source, Group) entries represents a Shortest Path Tree (SPT) that leads to the source of the multicast traffic. In sparse mode multicast routing, the root of the tree could actually be a central Rendezvous Point (RP) router rather than the actual traffic source. Because each router must know about the path back to the source or RP, the term Reverse Path Forwarding (RPF) is often used to describe the process of building the SPT.


Two important elements are required for a multicast network to work. The first we've already mentioned: you need a way to route multicast packets from the source to all of the various destinations in the group. The other critical element is that the multicast network has to provide a way for end devices to subscribe to a multicast group so that they can receive the data. The network uses the Internet Group Management Protocol (IGMP) to manage group subscriptions.


A multicast address is associated with a group of interested receivers. According to RFC 3171 , addresses to are designated as multicast addresses. This range was formerly called "Class D." The sender sends a single datagram (from the sender's unicast address) to the multicast address, and the routers take care of making copies and sending them to all receivers that have registered their interest in data from that sender.



Also See

IP Multicast