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24-14
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 24 Understanding and Configuring IP Multicast
Configuring IP Multicast Routing
When the switch populates the multicast routing table, dense-mode interfaces are always added to the
table. Sparse-mode interfaces are added to the table only when periodic join messages are received from
downstream routers, or when there is a directly connected member on the interface. When forwarding
from a LAN, sparse-mode operation occurs if there is an RP known for the group. If so, the packets are
encapsulated and sent toward the RP. When no RP is known, the packet is flooded in a dense-mode
fashion. If the multicast traffic from a specific source is sufficient, the receiver’s first-hop router can send
join messages toward the source to build a source-based distribution tree.
There is no default mode setting. By default, multicast routing is disabled on an interface.
Enabling Dense Mode
To configure PIM on an interface to be in dense mode, perform this task:
See the “PIM Dense Mode Example” section at the end of this chapter for an example of how to
configure a PIM interface in dense mode.
Enabling Sparse Mode
To configure PIM on an interface to be in sparse mode, perform this task:
See the “PIM Sparse Mode Example” section at the end of this chapter for an example of how to
configure a PIM interface in sparse mode.
Enabling Sparse-Dense Mode
When you enter either the ip pim sparse-mode or ip pim dense-mode command, sparseness or
denseness is applied to the interface as a whole. However, some environments might require PIM to run
in a single region in sparse mode for some groups and in dense mode for other groups.
An alternative to enabling only dense mode or only sparse mode is to enable sparse-dense mode. In this
case, the interface is treated as dense mode if the group is in dense mode; the interface is treated in sparse
mode if the group is in sparse mode. If you want to treat the group as a sparse group, and the interface
is in sparse-dense mode, you must have an RP.
If you configure sparse-dense mode, the idea of sparseness or denseness is applied to the group on the
switch, and the network manager should apply the same concept throughout the network.
Another benefit of sparse-dense mode is that Auto-RP information can be distributed in a dense-mode
manner; yet, multicast groups for user groups can be used in a sparse-mode manner. Thus, there is no
need to configure a default RP at the leaf routers.
Command Purpose
Switch(config-if)# ip pim dense-mode
Enables dense-mode PIM on the interface.
Command Purpose
Switch(config-if)# ip pim
sparse-mode
Enables sparse-mode PIM on the interface.