9.5 Configure IP Multicast Routing
9.5.2
Configuring PIM-DM
Configuring PIM-DM is really quite straightforward. On each router in the network, IP multicast routing must be enabled. Use the following global command (which was first introduced in IOS version 10.1):

Router(config)#ip multicast-routing

Then, it is best to enable PIM-DM on every interface in every router in the network, using the following interface command:

Router(config-if)#ip pim dense-mode

The reason for turning on multicast on every interface is that IP multicast routing is upside-down routing that forwards (or does not forward) multicast packets based on where the packets came from (source IP address) as opposed to where they are going (multicast group address). Consequently, the control mechanism for multicast traffic paths is not quite the same as the control mechanism for unicast routing. (see Figure )

Note: Actually, the recommended method for enabling multicast on an interface is the use of the ip pim sparse-dense-mode command. This command allows the router to use either dense or sparse mode, depending on the existence of RP information for each multicast group. This makes it much easier to switch the entire network from dense mode to sparse mode (or vice versa) as needed.

Not turning on multicast routing on every interface is a common mistake when network administrators are first enabling multicast routing. Failure to enable multicast on every interface often results in RPF failures because the RPF interface for a particular source of multicast traffic is the interface that would be used if the router sent traffic to this particular source. For example, Figure shows two routers within a network with parallel but unequal cost links.
Lab Activity
  In this lab activity, you will learn how to configure multicast routing on the Route Switch Module using Dense Mode Protocol-Independent Multicast.