Which of the following descriptions about SPT switching in PIM-SM is wrong?

Question

Options

• AThe RPT tree may not be the optimal path for multicast traffic forwarding
• BIf all multicast traffic passes through the RP, the RP may become the bottleneck of data forwarding
• CAfter the SPT switching is completed, the multicast traffic still passes through the RPT tree
• DThe SPT path is the shortest and the forwarding performance is better

• A
  6% (1)
• C
  81% (13)
• D
  13% (2)

Explanation

Option C is the incorrect statement because SPT switching exists precisely to replace the RPT (Rendezvous Point Tree) with the SPT (Shortest Path Tree). Once a last-hop router completes SPT switching, multicast traffic flows directly from the source to receivers via the SPT, and the RPT is no longer used for that traffic - the router sends a Prune message upstream to cut off the RPT flow.

Why A, B, and D are all correct (and therefore not the answer):

• A is true: the RPT routes all traffic through the RP, which may be geographically distant from the source or receivers, making it suboptimal.
• B is true: funneling all multicast traffic through a single RP creates a potential bottleneck, which is exactly why SPT switching was introduced.
• D is true: the SPT builds a path directly from source to each last-hop router, yielding the shortest possible route and better forwarding performance.

Memory tip: Think of SPT switching as a one-way door - once you walk through it (switch to SPT), you lock the RPT door behind you (Prune the RP path). Traffic never goes back through the RPT for that source-group pair.

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