Which two types of routing entries on a PIM network, (S, G) routing entries are used to set up on a PIM network and are applicable to both PIM-DM and PIM-SM modes.

Question

Options

• ASPT
• B
• C
• D

• A
  84% (26)
• B
  3% (1)
• C
  3% (1)
• D
  10% (3)

Explanation

(S, G) routing entries are used to build Shortest Path Trees (SPT), which establish a direct forwarding path from a specific multicast source (S) to a multicast group (G). SPT applies to both PIM-DM and PIM-SM: in PIM-DM, the flood-and-prune mechanism always produces (S, G) source-specific tree entries, while in PIM-SM, (S, G) SPT entries are created either from the start or after the router switches away from the shared tree (, G) once a traffic threshold is crossed. The other answer choices (B, C, D) are not listed in this question, so no distractor analysis is possible, but the key distinction to understand is that (, G) entries represent the shared tree used only in PIM-SM, whereas (S, G) SPT entries are the common thread across both modes.

Memory tip: The "S" in (S, G) stands for a specific Source, and a direct path to a specific source is by definition the Shortest Path - so (S, G) always maps to SPT.

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