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| 7.3 |  | Routing Multicast Traffic |
| 7.3.11 |  | Configure IP multicast routing |

Configuring IP Multicast Routing Process Overview
The IP multicast routing tasks are divided into basic and advanced tasks. This module covers the basic tasks that must be performed to set up a multicast session.

The first two basic tasks in the list are required to configure IP multicast routing; the remaining tasks are optional.

Configuring PIM-SM is really quite straightforward. On each router in the network, IP multicast routing must be enabled by using 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-SM on every interface in every router in the network, using the following interface command:

> Router(config-if)#ip pim sparse-mode

The final step is to configure each of the routers in the network with the IP address of the RP using the following global command:

> Router(config)#ip pim rp-address <address>

This command tells the router where to send the (*, G) joins when it needs to join the shared tree in sparse 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.

> Note: Another 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.