This article explains how  to rollback a software upgrade using a snapshot.

Before performing a software upgrade, you can create a snapshot of the running software in your device. This file will be stored in an alternate partition in the hard disk. 

The following example uses an ACX4000 running 12.3X54-D10.6:

You can also create a snapshot with the command, 'request system snapshot slice alternate'. This can be done as well with a USB:

Once you have created the snapshot, you can proceed with upgrade. In old releases, you cannot see the snapshot. But in recent releases, you can check the snapshot using the command 'show system snapshot'. The following is an example where the ACX has been upgraded to 16.1R7.7:

Check the snapshot to see the one created in 12.3:

From this point, if you want to rollback, you need to boot from the alternate partition. In other platforms, you can use the command 'request system reboot slice alternate media internal' to reboot from the alternate partition. However, in the ACX this is not possible. Therefore, you have to use the shell to make the ACX boot from the alternate partition.

1. First you need to login into the shell with root access:

   > start shell user root
Password:
%

2. Once in the shell, you can swap the active partition with the following commands:

   % fdisk -a da0

Partition 2 is marked active
Do you want to change the active partition? [n] y
Supply a decimal value for "active partition" [2] 1
Are you happy with this choice [n] y

%

3. Once partition '2', which is the ID for the alternate partition becomes active, you can proceed with the reboot. This can be done from the shell using the command, 'Reboot'.

   % reboot

Once the ACX boots, you will see that the router has booted with the version of the snapshot:

At this point, the ACX is running with the alternative partition as active. If you want to remain in the version of the snapshot and continue working with the primary partition, copy the snapshot to the primary partition to the primary partition using the command 'request system snapshot slice alternate'.

Once the snapshot has been copied in the primary partition, swap the active partition again.

At this point, the ACX has returned from 16.1R7.7 to 12.3X54, and is running from the primary partition.