Multicast PIM Route Pruning and SPT-Threshold

In PIM multicast routing, a specific (S,G) multicast route can be pruned when a router no longer requires that traffic due to the absence of interested hosts. This pruning occurs when a downstream router switches from the Rendezvous Point Tree (RPT) to the Shortest Path Tree (SPT) and sends a prune message to the upstream router.

The ip pim spt-threshold infinity command prevents a router from switching from the RPT to the SPT. This command is useful when the SPT and RPT are identical from the router's perspective. It does not affect pruning on other routers in the network.

To troubleshoot pruning issues, ensure that the appropriate router (e.g., the last-hop router) makes the switch from RPT to SPT for the relevant multicast group. This switch triggers the prune message, which results in the pruning of the (S,G) route on upstream routers.

https://networklessons.com/multicast/multicast-pim-sparse-mode