Is every source-based tree considered a shortest-path tree?

[Mitrixsen]

Hello, everyone.

When configuring Multicast, I often see the Source-Based tree called the Shortest-Path tree. It's basically the tree where the multicast sender (or the FHR connected to it) is the root of the tree and it involves the shortest/best path to reach a particular destination.

My question is, is every source-based tree also a shortest-path tree? In Sparse-Mode, between the FHRs and the RP, assuming that a multicast stream is active, a source-based tree is built which is used to forward the flow to the RP.

Is this also a shortest path tree? Because technically, it doesn't go straight to the destination LHR but to the RP instead so the path between the FHR and the LHR isn't the shortest or the most direct.

I hope that makes sense.

Thank you
David

[Joseph W. Doherty · Accepted solution]

The multicast path tree, using the RP for transit, might, or might not, also be the shortest path tree (between source and receiver), but if not, multicast can (optionally) dynamically switch over to a shortest path tree between source and receiver.  (I recall requiring using the RP for transit is termed using a shared tree.)

[sdroy · Accepted solution]

Not every source-based tree is a shortest-path tree, particularly in the context of Sparse-Mode Protocol Independent Multicast. While a source-based tree typically represents the most direct routes from a source (or FHR) to all destinations, the path between the FHR and the Rendezvous Point (RP) isn't necessarily the shortest or most direct route to the receivers (LHRs). This is because the RP acts as an intermediary in the multicast flow during the initial shared tree setup. Only when the multicast flow switches from the shared tree to a true source-based shortest-path tree does it achieve the direct, optimized route for multicast forwarding.