In the PIM router, the (S, G) routing table entry guides packet forwarding. If the received multicast packet does not exist in the (S, G) routing table entry, which of the following table entries…

Question

Options

• A(S,*)
• B(*,G)
• C(,)
• D(port)

• A
  3% (1)
• B
  69% (25)
• C
  8% (3)
• D
  19% (7)

Explanation

In PIM Sparse Mode, when a multicast packet arrives for group G from source S but no (S,G) entry exists, the router falls back to the (*,G) entry - the shared-tree state created when receivers joined group G via the Rendezvous Point (RP). The router uses this (*,G) entry to forward the packet and simultaneously creates a new (S,G) entry to optimize future forwarding along a source-specific tree.

Why the distractors are wrong:

• A. (S,*) - PIM does not use source-wildcard/any-group entries; multicast forwarding is group-driven, not source-driven.
• C. (*,*) - This double-wildcard form is not a standard PIM table entry used to seed (S,G) creation; no such fallback mechanism exists in PIM-SM.
• D. (port) - A port is a Layer 4 concept and has no role in IP multicast routing table lookups.

Memory tip: Think of (*,G) as the "group reservation" - receivers book seats for group G before any source shows up. When a source finally arrives, the router uses that pre-existing group reservation as the blueprint to build the specific (S,G) forwarding entry.

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