The PIM Assert mechanism is designed to prevent duplicate multicast packet forwarding on a multi-access network segment. The trigger for this process occurs when a PIM router receives a multicast data packet for a specific source-group pair (S,G) on an interface that is listed in its own outgoing interface (OIF) list for that same (S,G) entry. This event signifies that at least one other router on the same network segment is also forwarding the same traffic, prompting the routers to send Assert messages to elect a single forwarder and prune the others.

A. IGMP Queries are used by routers to discover which multicast groups have active members on a local network segment; they do not trigger PIM Assert messages.

B. PIM Hello messages are used for neighbor discovery and to elect a Designated Router (DR) on a shared segment, which is a different function from the Assert mechanism.

C. BSMs (Bootstrap Messages) are part of the PIM-SM Bootstrap Router (BSR) mechanism for distributing Rendezvous Point (RP) information, not for resolving duplicate data forwarding.

1. RFC 7761: Protocol Independent Multicast - Sparse Mode (PIM-SM), Section 4.5.1, "Sending PIM Assert Messages". The document states: "A PIM router sends an Assert message on a multi-access, multi-PIM-neighbor LAN when it receives a multicast data packet from a source S on an outgoing interface for an (S,G) entry." (DOI: https://doi.org/10.17487/RFC7761)

2. Nokia 7750 SR OS Multicast Routing Protocols Guide, Release 21.10.R1, Section: "PIM Assert". The guide explains: "The PIM assert mechanism is used to resolve the issue of receiving duplicate multicast packets. This can occur when more than one router is forwarding multicast traffic onto the same multi-access network." This confirms the scenario where a packet is received on an OIF.

3. Kurose, J. F., & Ross, K. W. (2017). Computer Networking: A Top-Down Approach (7th ed.). In Chapter 4, Section 4.8 on Multicasting, the PIM Assert mechanism is described as being initiated when a router detects that another router on the same broadcast LAN is forwarding the same multicast datagram, which it detects by receiving that datagram on its own outgoing interface.