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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter24 Configuring IP Multicast Routi ng Cisco Implementation of IP Multicast Routing
PIM SM
PIM SM uses shared trees and SPTs to distribute multicast traffic to multicast receivers in the network.
In PIM SM, a router or multilayer switch assumes that other routers or switches do not forward multicast
packets for a group, unless there is an explicit request for the traffic (join message). When a host joins
a multicast group using IGMP, its directly connected PIM SM device sends PIM join m es sag es tow ard
the root, also known as the RP. This join message travels router-by-router toward the root, constructing
a branch of the shared tree as it goes. The RP keeps t ra ck o f m ul tica s t r ec eiv er s; it also registers sources
through register messages received from the sources first-hop router (desi gnat ed router [DR] ) to
complete the shared tree path from the source to the receiver. The branches of the shared tree are
maintained by periodic join refresh messages that the PIM SM devices send along the branch.
When using a shared tree, sources must send their traffic to the RP so that the traffic reaches all receivers.
The special notation *,G, (pronounced star comma G) is used to represent the tree, where * means all
sources and G represents the multicast group. Figure 24-5 shows a shared tree for group 224.2.2.2 with
the RP located at Router 3. Multicast group traffic from source Hosts A and D travels to the RP
(Router3 ) and then down the shared tree to two receivers, Hosts B and C. Because all sources in the
multicast group use a common shared tree, the special notation (*, 224.2.2.2) describes this shared tree.
Figure24-5 Shared Distribution Tree
Note In addition to using the shared distribution tree, PIM SM can also use SPTs. By joining an SPT,
multicast traffic is routed directly to the receivers without having to go through the RP, thereby
reducing network latency and possible congestion at the RP. The disadvantage is tha t PI M SM
devices must create and maintain (S,G) state entries in their r outin g ta ble s al on g with the (S,G) SPT.
This action consumes router resources.
Host A
Source 1
194.1.1.1
194.2.2.2
Host B
Receiver
194.3.3.3
Host C
Receiver
Host D
Source 2
194.4.4.4
Router 1
DR Router 2
Router 5
Router 4
Router 3
RP
224.2.2.2 Traffic
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