How to Commit Your Junos Configuration
The Junos candidate configuration file is only the "proposed" configuration; your device does not use any of this configuration until you activate this candidate configuration using the commit command.
When you have definitely made all your changes, done all your checks, and are ready to make your candidate the active configuration running the device, enter the commit command:
 wiley@netnik# commit
As part of the commit process, Junos checks basic syntax and semantics. For example, the software makes sure that a policy has been defined before it is referenced. If any syntax or semantic problems are found, the commit command returns an error:
 wiley@netnik# commit error: Policy error: Policy my-policy referenced but not defined error: BGP: export list not applied error: configuration check-out failed
You must fix all mistakes before the candidate (or any part of the candidate) can become active.
When the activation is done, you see the commit complete message, which indicates that all the configuration statements of your candidate are now operating as the active configuration of your device:
 wiley@netnik# commit commit complete
Junos activation is a batch process — all the device configuration statements start operating only after you enter a commit statement, and all at the same time. In other systems, you might have to carefully consider the order in which you enter commands, because each command becomes immediately active as soon as you press Enter.
You don’t have this constraint with Junos. All configuration statements start to run at the same time without any delays.