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Software Configuration Guide—Release 15.0(2)SG
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Chapter 33 Configuring IP Multicast
About IP Multicast

PIM Sparse Mode

PIM Sparse Mode (PIM-SM) uses a pull model to deliver multicast traffic. Only networks with active
receivers that have explicitly requested the data are forwarded the traffic. PIM-SM is intended for
networks with several different multicasts, such as desktop video conferencing and collaborative
computing, that go to a small number of receivers and are typically in progress simultaneously.

Bidirectional PIM Mode

In bidirectional PIM (Bidir-PIM) mode, traffic is routed only along a bidirectional shared tree that is
rooted at the rendezvous point (RP) for the group. The IP address of the RP functions as a key enabling
all routers to establish a loop-free spanning tree topology rooted in that IP address.
Bidir-PIM is intended for many-to-many applications within individual PIM domains. Multicast groups
in bidirectional mode can scale to an arbitrary number of sources without incurring overhead due to the
number of sources.
For more detailed information on Bidirectional Mode, refer to this URL:
http://www.cisco.com/en/US/prod/collateral/iosswrel/ps6537/ps6552/ps6592/prod_white_paper0900ae
cd80310db2.pdf.
Rendezvous Point (RP)
If you configure PIM to operate in sparse mode, you must also choose one or more routers to be
rendezvous points (RPs). Senders to a multicast group use RPs to announce their presence. Receivers of
multicast packets use RPs to learn about new senders. You can configure Cisco IOS software so that
packets for a single multicast group can use one or more RPs.
The RP address is used by first hop routers to send PIM register messages on behalf of a host sending a
packet to the group. The RP address is also used by last hop routers to send PIM join and prune messages
to the RP to inform it about group membership. You must configure the RP address on all routers
(including the RP router).
A PIM router can be an RP for more than one group. Only one RP address can be used at a time within
a PIM domain for the same group. The conditions specified by the access list determine for which groups
the router is an RP (as different groups can have different RPs).
IGMP Snooping
IGMP snooping is used for multicasting in a Layer 2 switching environment. With IGMP snooping, a
Layer 3 switch or router examines Layer 3 information in the IGMP packets in transit between hosts and
a router. When the switch receives the IGMP Host Report from a host for a particular multicast group,
the switch adds the host's port number to the associated multicast table entry. When the switch receives
the IGMP Leave Group message from a host, it removes the host's port from the table entry.
Because IGMP control messages are transmitted as multicast packets, they are indistinguishable from
multicast data if only the Layer 2 header is examined. A switch running IGMP snooping examines every
multicast data packet to determine whether it contains any pertinent IGMP control information. If IGMP
snooping is implemented on a low end switch with a slow CPU, performance could be severely impacted
when data is transmitted at high rates. On the Catalyst 4500 series switches, IGMP snooping is
implemented in the forwarding ASIC, so it does not impact the forwarding rate.