The operator login class allows users to execute operational mode commands such as monitoring and troubleshooting commands, while preventing access to configuration mode for making or committing changes.

The Junos Command Line Interface (CLI) is strictly partitioned into operational and configuration modes to maintain a clear boundary between monitoring and administrative changes. Operational mode, identified by the > prompt, is designed for executing commands that monitor device status, troubleshoot network connectivity, and manage the physical system state. One typical administrative task performed here is clearing log files (using the clear log command) to free up storage space or reset diagnostic data during troubleshooting.
Additionally, high-level system requests that do not alter the persistent configuration database, such as rebooting the hardware, are executed in operational mode. The request system reboot command allows the administrator to gracefully restart the device after a software update or as part of a maintenance cycle. In contrast, actions such as configuring routing protocols or committing changes require transitioning to configuration mode (the # prompt). Committing a configuration is the specific act of moving changes from the candidate database to the active database, which is a logic-altering event strictly reserved for configuration mode. Therefore, operational mode remains the dedicated environment for real-time observation and system- level requests that ensure the device operates correctly within its existing parameters. Reference: User Interfaces, CLI Modes, Operational Mode Commands.

Class of Service (CoS) is a method used to prioritize traffic based on its type or importance. It allows you to define different levels of service for different types of traffic.
B). Traffic congestion can be managed: CoS helps to prioritize traffic and allocate bandwidth more effectively, allowing critical or time-sensitive traffic to be given priority over less important traffic, which helps to manage congestion.
D). Latency-sensitive traffic can be prioritized: CoS enables you to give higher priority to latency- sensitive traffic (e.g., VoIP or video) over less time-sensitive traffic (e.g., file transfers), ensuring that these critical services perform well even during network congestion.

The show | compare command (or show | compare rollback 0) highlights the differences between the candidate configuration and the currently active configuration, marking additions with a plus sign and deletions with a minus sign.

Area 0 is the OSPF backbone area, and all other areas are generally required to connect to it, either directly or through virtual links, to maintain proper route propagation.