Pim Shared Tree And Source Tree - Cisco Catalyst 3850 series Configuration Manual

Configuring PIM
Multicast Forwarding
Network designers must carefully consider the placement of the rendezvous point (RP) when implementing
a shared tree-only environment.
In unicast routing, traffic is routed through the network along a single path from the source to the destination
host. A unicast router does not consider the source address; it considers only the destination address and how
to forward the traffic toward that destination. The router scans through its routing table for the destination
address and then forwards a single copy of the unicast packet out the correct interface in the direction of the
destination.
In multicast forwarding, the source is sending traffic to an arbitrary group of hosts that are represented by a
multicast group address. The multicast router must determine which direction is the upstream direction (toward
the source) and which one is the downstream direction (or directions) toward the receivers. If there are multiple
downstream paths, the router replicates the packet and forwards it down the appropriate downstream paths
(best unicast route metric)--which is not necessarily all paths. Forwarding multicast traffic away from the
source, rather than to the receiver, is called Reverse Path Forwarding (RPF). RPF is described in the following
section.

PIM Shared Tree and Source Tree

By default, members of a group receive data from senders to the group across a single data-distribution tree
rooted at the RP.
The following figure shows this type of shared-distribution tree. Data from senders is delivered to the RP for
distribution to group members joined to the shared tree.
Figure 7: Shared Tree and Source Tree (Shortest-Path Tree)
If the data rate warrants, leaf routers (routers without any downstream connections) on the shared tree can
use the data distribution tree rooted at the source. This type of distribution tree is called a shortest-path tree
or source tree. By default, the software devices to a source tree upon receiving the first data packet from a
source.
This process describes the move from a shared tree to a source tree:
1 A receiver joins a group; leaf Router C sends a join message toward the RP.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
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