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Configuring IP Multicast Routing
Prerequisites
Figure 89 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 switches 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.
2. The RP puts a link to Router C in its outgoing interface list.
3. A source sends data; Router A encapsulates the data in a register message and sends it to the RP.
4. The RP forwards the data down the shared tree to Router C and sends a join message toward the source. At this
point, data might arrive twice at Router C, once encapsulated and once natively.
5. When data arrives natively (unencapsulated) at the RP, it 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
device 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.
Multiple sources sending to groups use the shared tree.
You can configure the PIM device to stay on the shared tree. For more information, see Delaying the Use of PIM
Shortest-Path Tree, page 710.
Prerequisites
To use multicast routing, the switch must be running the IP services image.
Router A
Source
Receiver
Router C
RP
Router B
Shared tree
from RP
Source tree
(shortest
path tree)
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