Cisco Systems 3560 Manual Do Utilizador

This process describes the move from a shared tree to a source tree:

A receiver joins a group; leaf Router C sends a join message toward the RP.

The RP puts a link to Router C in its outgoing interface list.

A source sends data; Router A encapsulates the data in a register message and sends it to the RP.

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.

When data arrives natively (unencapsulated) at the RP, it sends a register-stop message to Router A.

By default, reception of the first data packet prompts Router C to send a join message toward the 
source.

When Router C receives data on (S,G), it sends a prune message for the source up the shared tree.

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 the 

The change from shared to source tree happens when the first data packet arrives at the last-hop router 
(Router C in 

). This change occurs because the ip pim spt-threshold global configuration 

command controls that timing. 

The shortest-path tree requires more memory than the shared tree but reduces delay. You might want to 
postpone its use. Instead of allowing the leaf router to immediately move to the shortest-path tree, you 
can specify that the traffic must first reach a threshold. 

You can configure when a PIM leaf router should join the shortest-path tree for a specified group. If a 
source sends at a rate greater than or equal to the specified kb/s rate, the multilayer switch triggers a PIM 
join message toward the source to construct a source tree (shortest-path tree). If the traffic rate from the 
source drops below the threshold value, the leaf router switches back to the shared tree and sends a prune 
message toward the source.

You can specify to which groups the shortest-path tree threshold applies by using a group list (a standard 
access list). If a value of 0 is specified or if the group list is not used, the threshold applies to all groups.