Using Multicast in Your Network
Fixed Switch Configuration Guide 19-11
Figure 19-3 DVMRP Pruning and Grafting
Protocol Independent Multicast (PIM)
Overview
PIM dynamically builds a distribution tree for forwarding multicast data on a network. It is
designed for use where there may be many devices communicating at the same time, and any one
of the devices could be the sender at any particular time. Scenarios for using PIM multicasting
include desktop video conferencing and telephone conference calls.
PIM relies on IGMP technology to determine group memberships and uses existing unicast routes
to perform reverse path forwarding (RPF) checks, which are, essentially, a route lookup on the
source. Its routing engine then returns the best interface, regardless of how the routing table is
constructed. In this sense, PIM is independent of any routing protocol. It can perform RPF checks
using protocol-specific routes (for example, OSPF routes), static routes, or a combination of route
types.
PIM, a shared-tree technology, designates a router as the rendezvous point (RP), which is the root
of a shared tree for a particular group. All sources send packets to the group via the RP (that is,
traffic flows from the sender to the RP, and from the RP to the receiver). By maintaining one RP-
rooted tree instead of multiple source-rooted trees, bandwidth is conserved.
Figure 19-4 on page 19-12 illustrates the PIM traffic flow.
Multicast
Traffic
Prune*
Prune
Graft
IGMP Join
Source
New Host
DVMRP Multicast
Existing Host
Prune before new
host was added
*
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