| 9.5 | Configure IP Multicast Routing | ||
| 9.5.2 |
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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):
Then, it is best to enable PIM-DM on every interface in every router in the network, using the following interface command:
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
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