Been working through IP Multicast Routing on INE ATC video on demand section – I was trying to come up with acronyms in order to easily remember (S,G) and (*,G) and the shortest path tree vs. shared tree.
For (S,G) entries – these are the Source/Sender of the multicast packets and the Multicast Group destination. The RP builds the shortest path tree (SPT) from the Source to the RP.
For (*,G) entries – these are the Receiver/Join of the multicast packets and the Multicast Group destination. The RP builds the shared tree (RPT) from the RP to the Receiver(s).
S = Source/Sender – easy to remember – and we need to build the shortest path from the sender to the receiver.
* = Receiver (and since we’re coming off the Christmas season, the “Star” represents the destination of Bethlehem – where Jesus was born – and since I’m a born again Christian – I know I have to “receive” Jesus in order to be saved and I need to “share” my faith and the story.) Therefore, the Receiver is part of building the “shared” tree from the Receiver to the RP.
Lastly, the RP merges the shortest path tree and the shared tree in order to connect the Sender & Receiver. The RP may or may not be in the data plane path – depending on the topology of the network and the loop free path taken from the Sender to Receiver.
Also adding – if you are only building the shortest path tree and there is no mention of a shared tree – then use Dense or Sparse-Dense mode and no configuration of an RP is required. Dense or Sparse-Dense mode limits you to building either a source-based distribution tree or a shortest path tree.
Sparse mode builds both a shared multicast distribution tree and a source based distribution tree.
Dense uses flood & prune mechanism, Sparse saves bandwidth by only connecting sender to the receiver.
That’s my $0.02 for today – no apologies from me for inserting my faith into my CCIE studies – I know my faith can only help me on this journey.
padawanccie 