To install software upgrades on a router using the CLI:
Replace source with one of the following:
The validate option validates the software package against the current configuration as a prerequisite to adding the software package to ensure that the router reboots successfully. This is the default behavior when the software package being added is a different release.
The unlink option removes the package at the earliest opportunity in order to make room to complete the installation.
Adding the reboot command reboots the router after the upgrade is validated and installed. When the reboot is complete, the router displays the login prompt.
Rebooting occurs only if the upgrade is successful.