Multicast Routing/ How does Multicast routing work?/ What are multicast routing protocols?

Hello Friends, in this blog post I am going to let you know about the multicast routing. The procedure of sending a message to a group of widely separated processes in a network is called multicasting.

The message is sent to well-defined groups that are numerically large in size but small as compared to the network, as a whole. An example of where different processes work together in groups in a distributed database system. The routing algorithm is called multicasting routing.

multicasting requires group management to create and destroy groups and for processes to leave and join groups. multicast routing is concerned with the fact that when a process joins a group, it must inform its host.

It is important that routers must know which of their hosts belong to which group. As the routers tell their neighbors, the information propagates
through the subnet.

In multicast routing, each router computes a spanning tree for itself, covering all other routers in the subnet. Let us consider an example given in fig 1.

In fig1 a, a subnet with two groups 1 and 2 is given. Some routers are attached to hosts that belong to one or both of these groups. A spanning tree for the left-most routers is shown in fig1 b.

When a process sends a multicast packet to a group, the first router of the subnet forms a pruned spanning tree, removing all lines not leading to the hosts of the groups.

In fig1c and fig1d pruned spanning trees for group 1 and group 2 are shown. Multicast packets are forwarded only along the appropriate spanning tree.

There are various algorithms used to prune the spanning tree; like link-state routing, distance vector routing, reverse path forwarding, etc.

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