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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter24 Configuring IP Multicast Routi ng Configuring Advanced PIM Features
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 m essage and sends i t to t he 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 n ot se nt
hop-by-hop. They are sent by the designated router that is directly connecte d 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 “Delaying
the Use of PIM Shortest-Path Tree” section on page 24-29.
Delaying the Use of PIM Shortest-Path Tree
The change from shared to source tree happen s w hen t he fir st dat a pa ck et ar ri ves at the l as t-h op rou ter
(Router C in Figure 24-9). This change occurs because the ip pim spt-threshold interface configuration
command controls that timing; its default setting is 0 kbps.
The shortest-path tree requires more memory than the shared tree b u t reduces delay. You might want to
postpone its use. Instead of allowing the leaf router to im medi at ely mo ve t o the shor test- pa th t ree , 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 kbps 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 share d tre e and send s 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.