33-33
Cisco ME 3800X and 3600X Switch Software Configuration Guide
OL-23400-01
Chapter 33 Configuring IP Multicast Routing
Configuring Advanced PIM Features
Configuring Advanced PIM Features
• Understanding PIM Shared Tree and Source Tree, page 33-33
• Delaying the Use of PIM Shortest-Path Tree, page 33-34 (optional)
• Modifying the PIM Router-Query Message Interval, page 33-35 (optional)
Understanding 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. Figure 33-5 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 33-5 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.
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