V-251397

Discussion

Any Source Multicast (ASM) can have many sources for the same groups (many-to-many). For many receivers, the path via the Rendezvous Point (RP) may not be ideal compared with the shortest path from the source to the receiver. By default, the last-hop router will initiate a switch from the shared tree to a source-specific shortest-path tree (SPT) to obtain lower latencies. This is accomplished by the last-hop router sending an (S, G) PIM Join towards S (the source). When the last-hop router begins to receive traffic for the group from the source via the SPT, it will send a PIM Prune message to the RP for the (S, G). The RP will then send a Prune message towards the source. The SPT switchover becomes a scaling issue for large multicast topologies that have many receivers and many sources for many groups because (S, G) entries require more memory than (*, G). Hence, it is imperative to minimize the amount of (S, G) state to be maintained by increasing the threshold that determines when the SPT switchover occurs.

Check Content

Review the multicast last-hop router configuration to verify that the SPT switchover threshold is increased (default is 0) or set to infinity (never switch over). The following is a PIM sparse mode last-hop router configuration example that will disable the SPT switchover for all multicast groups:

ip multicast-routing
ip pim spt-threshold infinity

If any multicast router is not configured to increase the SPT threshold or set it to infinity to minimalize (S,G) state, this is a finding.