Device Switchover Upon Failure - AudioCodes Mediant 4000 SBC User Manual

Note: If the active unit runs an earlier version (e.g., 7.0) than the redundant unit (e.g.,
7.2), the redundant unit is downgraded to the same version as the active unit (e.g.,
7.0).
Thus, under normal operation, one of the devices is in active state while the other is in
redundant state, where both devices share the same configuration and software. Any
subsequent configuration update or software upgrade on the active device is also done on
the redundant device.
In the active device, all logical interfaces (i.e., Media, Control, OAMP, and Maintenance)
are active. In the redundant device, only the Maintenance interface is active, which is used
for connectivity to the active device. Therefore, management is done only through the
active device. Upon a failure in the active device, the redundant device becomes active
and activates all its logical interfaces exactly as was used on the active device.
32.2

Device Switchover upon Failure

When a failure occurs in the active device, a switchover occurs to the redundant device
making it the new active device. Whether a switchover is later done back to the repaired
failed device, depends on whether you have enabled the Preempt mode:

Enabled: The Preempt mode specifies one of the device's as the "preferred" device.
This is done by assigning different priority levels (1 to 10, where 1 is the lowest) to the
two devices. Typically, you would configure the active device with a higher priority
level (number) than the redundant device. The only factor that influences the
configuration is which device has the greater number; the actual number is not
important. For example, configuring the active with 5 and redundant with 4, or active
with 9 and redundant with 2 both assign highest priority to the active device.
Whenever the device with higher priority recovers from a failure, it first becomes the
redundant device but then initiates a switchover to become the active device once
again; otherwise, after recovery, it becomes the redundant device and remains as
redundant. If you change the priority level of the redundant device to one that is higher
than the active device and then reset the redundant device, a switchover occurs to the
redundant device making it the active device and the "preferred" device. If both
devices are configured with the same priority level, Preempt mode is disabled. Please
see note below when using priority level 10.

Disabled: A switchover is done only upon failure of the currently active device.
Failure detection by the devices is done by the constant keep-alive messages they send
between themselves to verify connectivity. Upon detection of a failure in one of the devices,
the following occurs:

Failure in active device: The redundant device initiates a switchover. The failed
device resets and the previously redundant device becomes the active device in
stand-alone mode. If at a later stage this newly active device detects that the failed
device has been repaired, the system returns to HA mode. If Preempt mode is
enabled and the originally active device was configured with a higher priority, a
switchover occurs to this device; otherwise, if it was configured with a lower priority (or
Preempt mode was disabled), the repaired device is initialized as the redundant
device.

Failure in redundant device: The active device moves itself into stand-alone mode
until the redundant device is returned to operation. If the failure in the redundant
device is repaired after reset, it's initialized as the redundant device once again and
the system returns to HA mode.
User's Manual 602
Mediant 4000 SBC
Document #: LTRT-40203