Which protocol is described in RFC 7761 and uses shared distribution trees rooted at the RP, but may switch to the source-rooted distribution tree?
- PIM-DM
- OSPFv3
- MP-BGP with MSDP
- PIM-SM
Explanation: Protocol Independent Multicast - Sparse Mode (PIM-SM) is the specific protocol described in RFC 7761. PIM-SM operates by using shared distribution trees that are rooted at a Rendezvous Point (RP) to coordinate the forwarding of multicast traffic from a source to its receivers.
A key characteristic of PIM-SM is that while it starts with a shared tree, it may switch to a source-rooted distribution tree (also known as a Shortest Path Tree or SPT). This transition typically occurs when a last-hop router exceeds a configured SPT threshold, allowing it to join the SPT and receive traffic directly from the source to reduce latency.
In contrast, PIM-DM (Dense Mode) uses a "push" model that only supports source trees and is incapable of building shared distribution trees rooted at an RP. OSPFv3 is a unicast routing protocol that can be used to populate the routing table but does not manage multicast distribution trees itself.
Related exam: Modules 8 – 13: Checkpoint Exam: Network Routing Answers (CCNP ENCOR v9)
