Monday, November 17, 2008

PIM SM - SPT Threshold

In PIM Sparse mode, SPT Thresholds can be configured to control when to switch to the Shortest Path Tree (SPT) or Source Tree.

The command in IOS is:

ip pim spt-threshold {kbps | infinity} [group-list access-list-number] 

Default is 0kbps, which means that all sources are switched over to SPT immediately.  By switching to SPT, usually you will get the most optimal path to deliver the multicast traffic.

If you use the “infinity” keyword, the source will not be switched to SPT and remain on the shared tree.  In networks with large numbers of senders, like video surveillance cameras, you will need extra resources on routers to keep track of the increased amount of states.  Using the “infinity” keyword can force certain groups to remain on shared tree when latency is not a big issue.

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