Juniper Junos Fundamentals

Junos OS gives you one operating model for many Juniper network devices. You use it to inspect device state, define configuration, and control how routes reach the forwarding hardware.

The central idea is separation. The Routing Engine runs control and management processes. The Packet Forwarding Engine moves packets between interfaces. The Routing Engine selects active routes and supplies a forwarding table to the Packet Forwarding Engine.

This split helps you reason about a fault. A routing protocol can fail to learn a route in the control plane. A learned route can fail to become active. An active route can fail to reach the forwarding table. Packets can still fail because an interface or next hop is unavailable. Each stage needs a different check.

Two CLI modes

Operational mode answers, “What is the device doing?” Its prompt ends with >. Commands such as show, ping, and traceroute display state or test connectivity.