The change from shared to
source tree happens when the first data packet arrives at the last-hop router.
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
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
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.
This procedure is optional.