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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 38 Configuring IP Multicast
Configuring IP Multicast Routing
Enabling PIM on an Interface
Enabling PIM on an interface also enables IGMP operation on that interface. An interface can be
configured to be in dense mode, sparse mode, or sparse-dense mode. The mode determines how the
Layer 3 switch or router populates its multicast routing table and how the Layer 3 switch or router
forwards multicast packets it receives from its directly connected LANs. You must enable PIM in one of
these modes for an interface to perform 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, enter this command:
For an example of how to configure a PIM interface in dense mode, see the “PIM Dense Mode Example”
section.
Enabling Sparse Mode
To configure PIM on an interface to be in sparse mode, enter this command:
For an example of how to configure a PIM interface in sparse mode, see the “PIM Sparse Mode
Example” section.
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. 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.
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.
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