What are the first two actions a router performs after PIM is enabled?

When Protocol Independent Multicast (PIM) is enabled on a router's interface, the first action is to establish neighbor relationships on the local network segment. This is accomplished by sending and receiving PIM Hello messages. Once PIM neighbors are discovered through this process, the routers on a multi-access segment proceed with the second action: the Designated Router (DR) election. The DR election uses information advertised in the Hello messages (such as DR priority and IP address) to select a single router responsible for sending PIM Join/Prune messages upstream toward the Rendezvous Point (RP). This sequence is fundamental for PIM operation on any shared network.

A, C, D: BSR (Bootstrap Router) election is a domain-wide process, not an initial per-interface action. It cannot precede the fundamental neighbor discovery and DR election on a local link.

B: The Designated Router (DR) election cannot occur before PIM Hello messages are exchanged, as the election process depends on information carried within those messages.

E: While BSR election is an important PIM-SM process, the DR election is the more immediate, per-segment action that directly follows neighbor discovery via Hello messages.

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

Section 4.3.1, "Hello Message": "PIM-SM uses Hello messages to discover PIM-SM neighbors." This establishes Hello messages as the mechanism for neighbor discovery.

Section 4.3.3, "Designated Router (DR) Election": "The DR is elected based on the DR priority and IP address advertised in PIM Hello messages." This explicitly states that the DR election process relies on information from Hello messages, confirming that Hellos must be exchanged first.

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

Section: PIM Overview: The guide describes how PIM routers use a Hello protocol to discover neighbors and that on multi-access networks, a Designated Router is elected to send messages on behalf of hosts. The operational logic described follows the sequence of neighbor discovery preceding the DR election. For instance, the "PIM Designated Router" subsection notes the election criteria (priority, IP address), which are communicated via the Hello protocol.

3. Fall 2018, CS 640: Computer Networks, University of Wisconsin-Madison Courseware

Lecture 21: IP Multicast: The lecture slides explain the PIM-SM mechanism, stating, "PIM routers on a LAN elect a designated router (DR)... Routers send Hello packets on their PIM-enabled interfaces to discover each other and elect the DR." This academic source confirms the sequence of Hellos for discovery, followed by DR election.