Junos OS Fundamentals and User Interfaces
This domain tests Junos OS architecture and user interfaces: the separation of control and forwarding planes, the candidate configuration model, commit and rollback operations, and the CLI modes used on Juniper data center switches.
Architecture and modes
Junos runs routing protocols and management on the Routing Engine (control plane) and forwards packets in the Packet Forwarding Engine (data plane). Operational mode starts with show, monitor, ping, or clear; configuration mode starts with configure and uses a candidate configuration. You edit with set/delete or edit blocks, inspect with show | compare, validate with commit check, and activate with commit. commit confirmed N applies the change temporarily and automatically rolls back unless you confirm within N minutes.
Worked example
An engineer adds a description to ge-0/0/1:
configure set interfaces ge-0/0/1 description "rack1-server" show | compare commit check commit confirmed 5
After verifying with show interfaces descriptions ge-0/0/1, the engineer runs commit to make the change permanent. If the link had failed, the automatic rollback would restore the previous configuration after five minutes without a second commit.
Traps
Forgetting to commit leaves changes only in the candidate. commit check validates syntax but does not activate. rollback 0 loads the active configuration into the candidate; rollback 1 loads the previous committed configuration. Do not use commit confirmed without a follow-up commit unless you intend an automatic revert. Treat set commands as candidate edits, not live changes.