IP PIM Sparse-Dense Mode
The name of this command gives an indication of the functionality it provides. Due to the
increasing use of multicast and the variety of applications available today, it is best to configure
an interface to be able to use both sparse mode and dense mode. With the previous commands,
the interface was assigned the operating mode, and the interface could not change between
modes depending on the need at the time.
PIM sparse-dense mode configuration now enables the interface to use whichever forwarding
method is needed by the application or multicast group. The interface uses the multicast group
notation to decide which mode it needs to operate in. If the interface sees something with the notation
(S, G), it operates in dense mode. If the interface sees a notation similar to (∗, G), the interface
operates in sparse mode.
An added benefit of implementing sparse-dense mode on the interfaces is the elimination of
the need to hard-configure the RP at every leaf router. The Auto-RP information is sent out
across the network by using dense mode forwarding.
Configuring IP Multicast Routing 625
IP PIM sparse-dense mode is enabled by using ip pim sparse-dense-mode on the interface
command line. Here is an example:
Enter configuration commands, one per line. End with CNTL/Z.
Terry_3640(config-if)#ip pim sparse-dense-mode
Again, here is what the interface looks like after the preceding lines have been executed:
ip address 172.16.21.4 255.255.255.0
no ip directed-broadcast
ip pim sparse-dense-mode
In summary, when using the sparse-dense mode configuration on an interface, you need to
understand that three criteria will activate the interface and place it into the multicast forwarding
table. The first criterion applies to either sparse or dense mode; the others cause the interface
to operate specifically for sparse or dense mode. Table 19.3 provides the details.