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

Preventing Use of Shortest-Path Tree to Reduce Memory Requirement

PIM Shared Tree and Source Tree - Shortest-Path Tree

By default, members of a multicast group receive data from senders to the group across a single data distribution
tree rooted at the rendezvous point (RP). This type of distribution tree is called shared tree, as shown in the
figure. Data from senders is delivered to the RP for distribution to group members joined to the shared tree.
Figure 16: Shared Tree versus Source Tree (Shortest-Path Tree)
If the data rate warrants, leaf routers on the shared tree may initiate a switch to the data distribution tree rooted
at the source. This type of distribution tree is called a shortest-path tree (SPT) or source tree. By default, the
software switches to a source tree upon receiving the first data packet from a source.
The following process describes the move from shared tree to source tree in more detail:
1 Receiver joins a group; leaf Router C sends a Join message toward the RP.
2 The RP puts the link to Router C in its outgoing interface list.
3 Source sends data; Router A encapsulates data in a register message and sends it to the RP.
4 The RP forwards data down the shared tree to Router C and sends a Join message toward the source. At
this point, data may arrive twice at Router C, once encapsulated and once natively.
5 When data arrives natively (through multicast) at the RP, the RP sends a register-stop message to Router
A.
6 By default, reception of the first data packet prompts Router C to send a Join message toward the source.
7 When Router C receives data on (S, G), it sends a Prune message for the source up the shared tree.
8 The RP deletes the link to Router C from the outgoing interface of (S, G). The RP triggers a Prune message
toward the source.
Join and Prune messages are sent for sources and RPs. They are sent hop-by-hop and are processed by each
PIM router along the path to the source or RP. Register and register-stop messages are not sent hop-by-hop.
They are sent by the designated router that is directly connected to a source and are received by the RP for
the group.
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3.6E (Catalyst 3850 Switches)
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