In PIM-DM, the assertion mechanism is to ensure that only one multicast router forwards multicast packets in the shared network segment. Which of the following parameters are the conditions for…

Question

Options

• AInterface IP address
• BRouter ID
• CDR priority
• DUnicast routing priority

• A
  75% (43)
• B
  25% (14)

Explanation

In PIM-DM, when multiple routers on the same segment receive the same multicast traffic, the Assert mechanism elects a single forwarder by comparing three parameters in order: unicast routing priority (D) - the router with the lower administrative distance to the source wins; DR priority (C) - a higher DR priority breaks a tie; and interface IP address (A) - the highest IP address is the final tiebreaker. Together, these three conditions uniquely identify the Assert Winner without ambiguity.

Why B (Router ID) is wrong: Router ID is an identifier used in routing protocols like OSPF for adjacency and LSA origination - it plays no role in PIM Assert message comparison. It is a common distractor because it sounds like a router selection parameter.

Memory tip: Think of the Assert election as a three-round knockout - "Admin distance, then DR priority, then Address" - or use the mnemonic "DDA" (Distance → DR priority → Address). When two values tie at one round, the next rule steps in, and Router ID never enters the ring.

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