Configuring the PIM SPT Threshold Policy
Multicast routing devices running PIM sparse mode can forward the same stream of multicast packets onto the same LAN through a rendezvous-point tree (RPT) rooted at the RP or through a shortest-path tree (SPT) rooted at the source. In some cases, the last-hop routing device needs to stay on the shared RPT to the RP and not transition to a direct SPT to the source. For more information about these SPT cutover cases, see SPT Cutover Control.
You configure an SPT threshold policy on the last-hop routing device to control the transition to a direct SPT. An SPT cutover threshold of infinity applied to a source-group address pair means the last-hop routing device never transitions to a direct SPT. For all other source-group address pairs, the last-hop routing device transitions immediately to a direct SPT rooted at the source DR.
To configure the SPT threshold and policy for PIM sparse mode, include the spt-threshold statement:
For a list of the hierarchy levels at which you can include this statement, see the statement summary section for this statement.
 | Note: If you want the PE router to stay on the RPT for control traffic, include the spt-threshold statement under the main PIM instance. |
The infinity statement must reference a properly configured policy to set the SPT cutover threshold for a particular source-group pair to infinity. The use of values other than infinity for the SPT threshold is not supported. You can configure more than one policy.
Several points are important when configuring the SPT threshold policy, as discussed in the following sections:
SPT Threshold Policy Configuration Changes
Configuration changes to the SPT threshold policy affect how the routing device handles the SPT transition:
- When the policy is configured for the first time, the
routing device continues to transition to the direct SPT for the source-group
address pair until the PIM-join state is cleared with the clear
pim join command.
Note: If you do not clear the PIM-join state when you apply the infinity policy configuration for the first time, you must apply it before the PE router is brought up.
- When the policy is deleted for a source-group address pair for the first time, the routing device does not transition to the direct SPT for that source-group address pair until the PIM-join state is cleared with the clear pim join command.
- When the policy is changed for a source-group address pair for the first time, the routing device does not use the new policy until the PIM-join state is cleared with the clear pim join command.
Examples of SPT Threshold Policy Configuration
The simplest type of SPT threshold policy uses a route filter and source address filter to specify the multicast group and source addresses and to set the SPT threshold for that pair of addresses to infinity. The policy is applied to the main PIM instance.
This example sets the SPT transition value for the source-group pair 10.10.10.1 and 224.1.1.1 to infinity. When the policy is applied to the last-hop router, multicast traffic from this source-group pair will never transition to a direct SPT to the source. Traffic will continue to arrive through the RP. However, traffic for any other source-group address combination at this router will transition to a direct SPT to the source.
