What is SPT & Shared Tree

In multicast we have shared tree and shortest path tree(SPT). Shared tree means alwasy your request will be processed by RP. It means whenever you want to reach any group you require to send your information to RP and RP will process it. In this type of scenarios *,G entry is created becasue you will not get the source address.
But in case of SPT a first *,G entry is created after that RP sends the group information to source and S,G entry is created. S,G means client knows which source is responsible for which multicast group. After that client can go to source with the help of local routing table. You can say RP is working as mediator first time and after that it comes out from the picture. But in case of shared tree it always works as mediator.
During design phase you have to decide which tree you are looking for. By default on cisco platform spt value is 0 means it will alwasy converge to SPT.


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