Cisco Systems 3750E manual Delaying the Use of PIM Shortest-Path Tree, 42-27

Chapter 42 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 42-5). This change occurs because the ip pim spt-thresholdglobal 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-listaccess-list-number {deny	Create a standard access list.
	permit} source [source-wildcard]	• 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}	Specify the threshold that must be reached before moving to
	[group-listaccess-list-number]	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-listaccess-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.

	Catalyst 3750-E and 3560-E Switch Software Configuration Guide
OL-9775-02		42-27