Re: Upgrading M7I from old version of 11.4R5.7 to 15.1R6
As per the below doc, you can To upgrade from EEOL release that occurs directly before or after the currently installed EEOL release or to two EEOL releases before or after. So from 11.4R5.7 to 15.1R6, we have the below releases as EEOL as per the link https://www.juniper.net/support/eol/junos.html , these are the EEOL releases from 11.4 to 15.1
11.4 , 12.3 , 13.3, 14.1, 14.2, 15.1
So you can jump two EEOL releases , so that would be from 11.4 to 13.3 . Then from 13.3 to 14.2 . then from 14.2 to 15.1 .
Upgrade and Downgrade Support Policy for Junos OS Releases
Support for upgrades and downgrades that span more than three Junos OS releases at a time is not provided, except for releases that are designated as Extended End-of-Life (EEOL) releases. EEOL releases provide direct upgrade and downgrade paths—you can upgrade directly from one EEOL release to the next EEOL release even though EEOL releases generally occur in increments beyond three releases.
You can upgrade or downgrade to the EEOL release that occurs directly before or after the currently installed EEOL release, or to two EEOL releases before or after. For example, Junos OS Releases 14.1, 14.2, 15.1 and 16.1 are EEOL releases. You can upgrade from Junos OS Release 14.1 to Release 15.1 or from Junos OS Release 15.1 to Release 16.2. However, you cannot upgrade directly from a non-EEOL release that is more than three releases ahead or behind.
To upgrade or downgrade from a non-EEOL release to a release more than three releases before or after, first upgrade to the next EEOL release and then upgrade or downgrade from that EEOL release to your target release.