SPT Switchover

Dec 09,2010 by admin

SPT Switchover


	A threshold on a leaf router can be configured that, when exceeded, will cause the router to switch from the shared tree through the RP to the source tree. When PIM-SM is enabled, the default threshold is 0 kbps. This means that when the first packet is received from a multicast source, the router switches from the shared tree to the source tree. The threshold can be configured from 0 to infinity. A setting of infinity prevents the router from ever switching to the source tree.

The traffic for the group G from any source S is measured once each second. If the threshold is exceeded, then set a flag for (*,G) to remember that the threshold was exceeded. When the next packet for G arrives from any source, if the threshold exceeded flag is set, then clear the flag in (*,G), set the flag in (S,G), and switch to the source tree for that particular source. Again, every second the state of the flag is in (S,G) will be checked and if the traffic rate is less than the threshold, then switch back to the shared tree. The advantages of switching to the source tree is that traffic is being received on the shortest path tree. The shortest path tree will generally have a lower latency than the shared tree. The disadvantage is that (S,G) state will have to be maintained in the router. In other words, there is more detail that has to be maintained.