Cisco Systems 3560-X, 3750-X Delaying the Use of PIM Shortest-Path Tree

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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 48 Configuring IP Multicast Routing Configuring Advanced PIM Features
Delaying the Use of PIM Shortest-Path Tree
The change from shared to source tree happens when the first data packet arrives at the last-hop router
(Router C in Figure 48-6). 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 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 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.
Beginning in privileged EXEC mode, follow these steps to configure a traffic rate threshold that must be
reached before multicast routing is switched from the source tree to the shortest-path tree. This
procedure is optional.
Command Purpose
Step 1 configure terminal Enter global configuration mode.
Step 2 access-list access-list-number {deny |
permit} source [source-wildcard]
Create a standard access list.
• For access-list-number, the range is 1 to 99.
• The deny keyword denies access if the conditions are matched.
The permit keyword permits access if the conditions are
matched.
• For source, specify the multicast group to which the threshold
will apply.
• (Optional) For source-wildcard, enter the wildcard bits in dotted
decimal notation to be applied to the source. Place ones in the
bit positions that you want to ignore.
Recall that the access list is always terminated by an implicit deny
statement for everything.
Step 3 ip pim spt-threshold {kbps | infinity}
[group-list access-list-number]
Specify the threshold that must be reached before moving to
shortest-path tree (spt).
• For kbps, specify the traffic rate in kilobits per second. The
default is 0 kbps.
Note Because of switch hardware limitations, 0 kbps is the only
valid entry even though the range is 0 to 4294967.
• Specify infinity if you want all sources for the specified group
to use the shared tree, never switching to the source tree.
• (Optional) For group-list access-list-number, specify the access
list created in Step 2. If the value is 0 or if the group-list is not
used, the threshold applies to all groups.