The most direct and common method for monitoring the health and status of a Link Aggregation Group (LAG) is by using the show interfaces command. On Junos devices, show interfaces ae provides detailed output about the logical LAG interface, including its status, traffic statistics, and, crucially, the status of all its physical member links. This allows an administrator to quickly verify if the bundle is up and which member links are actively participating in the aggregation.

B. The ping command is a Layer 3 tool used to test reachability, not a direct method for verifying Layer 2 LAG health.

C. A router's routing table reflects Layer 3 forwarding decisions; it is an indirect and lagging indicator of a LAG's status, not a direct monitoring tool.

D. While BFD can be run over a LAG to monitor the path, LACP is the protocol that monitors member links. The primary tool to check this status is the show command.

1. Juniper Networks TechLibrary

"Monitoring Aggregated Ethernet Interfaces". This document explicitly lists show interfaces ae-interface-name [extensive | detail] as the primary command for monitoring LAGs. It details how the output shows link status and LACP information for the bundle and its members.

2. Juniper Networks TechLibrary

"Aggregated Ethernet Interfaces Overview". Section "Monitoring an Aggregated Ethernet Interface" states

"To display the state of an aggregated Ethernet interface

use the show interfaces command."