Upon enabling Protocol Independent Multicast (PIM) on an interface, a router's first action is to send PIM Hello messages to discover other PIM-enabled routers on the same network segment. This establishes PIM neighbor adjacencies. Once neighbors are discovered on a multi-access segment (like Ethernet), they conduct a Designated Router (DR) election. The DR election process uses the priority and IP address information advertised in the Hello messages to select a single router responsible for registering sources and forwarding Join/Prune messages toward the Rendezvous Point (RP). This sequence ensures that a stable local PIM topology is established before more complex multicast routing functions are performed.

A. BSR and DR elections require neighbor adjacencies, which are established by PIM Hello messages first.

B. The DR election process relies on information exchanged in PIM Hello messages, so it cannot occur first.

C. Both DR and BSR elections are subsequent actions that depend on the initial neighbor discovery via PIM Hellos.

D. BSR elections are a domain-wide process that requires an established PIM topology, which begins with Hello messages.

E. While BSR elections follow Hellos, the DR election is a more immediate, per-segment action that also directly follows neighbor discovery.

1. RFC 7761: Protocol Independent Multicast - Sparse Mode (PIM-SM)

Section 4.2.1, "Sending Hello Messages": This section details that PIM routers periodically send Hello messages on PIM-enabled interfaces to discover neighbors. This is the foundational step.

Section 4.2.3, "Designated Router Election": This section explicitly states, "The election process is simple: every PIM router on the multi-access network sources a Hello message... These Hello messages contain the router's DR priority... The router with the highest DR priority is the DR." This confirms the DR election directly follows and uses information from the Hello messages.

2. Nokia 7750 SR OS R21.10.R1, Multicast Routing Protocols Guide

Section: PIM Overview: The document describes that PIM uses Hello messages to "discover new neighbors and to learn of existing neighbors that have failed." It then describes the Designated Router (DR) function, which is elected on multi-access networks. The logical dependency is that neighbor discovery must precede the election.

3. RFC 5059: Bootstrap Router (BSR) Mechanism for PIM

Section 3, "BSR Election": This RFC describes how Bootstrap messages are "flooded hop-by-hop throughout the PIM domain." This flooding relies on the underlying PIM adjacencies established by Hello messages, confirming that BSR elections occur after neighbor discovery is complete.