Alcatel-Lucent 7450 ESS-12 Configuration Manual page 58

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impacts to the running services are minimized during a rollback:

no impact in areas of configuration that did not change

configuration parameters that changed (or items that changed config have
dependencies on) are first removed (revert to default) and the previous values are then
restored (can be briefly service impacting in changed areas). Some examples are the
following:

If the currently active config contains "configure port 5/1/1 dwdm tdcm
dispersion -1000" and the rollback checkpoint contains "configure port 5/1/1
dwdm tdcm dispersion -1010", then the operational dispersion will transition from
-1000, to 0 and then back to -1010 for port 5/1/1 which will cause a traffic
interruption.

Changing the neighbor of a MC-APS port will start with neighbor 1, then be
configured as no neighbor, and then will be configured with neighbor 2. Moving
through the 'no neighbor' intermediate state requires the working and protect
circuits to be torn down and then rebuilt.
A rollback will undo any SNMP sets or direct CLI config commands that occurred since
the last checkpoint creation.
During the period when an SR-OS node is processing a rollback revert, both CLI
commands (from other users) and SNMP commands will continue to be processed. The
only commands that are blocked during a rollback revert are other rollback commands
including revert, save, and compare (only one rollback command can be executing at a
time in one node).
Commands are available to view and compare the various rollback checkpoints to current
operating and candidate configurations.
Rollback checkpoint files are not guaranteed to be in any particular format. They are not
interchangeable with normal config files or exec scripts. A normal config file (from an
admin save) cannot be renamed as a rollback checkpoint and then referenced for a
rollback revert operation. Only rollback checkpoint files generated with rollback save can
be used to rollback revert.
If a hardware change is made after a rollback-save then:

a rollback can be executed as long as the hardware change was an addition of
hardware to the node (for example, added a new IOM into a previously empty slot).

a rollback is not guaranteed to work if hardware was removed or changed (for
example, IOM was removed, or MDA was swapped for a different MDA type).
Rollback across a change to the following parameters is not supported:

chassis-mode

mixed-mode

the sr|ess capability of a card ('configure card capability sr|ess')
7450 ESS OS Basic System Configuration Guide