Device Switchover Upon Failure - AudioCodes Mediant 4000 SBC User Manual

CHAPTER 31 HA Overview
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.
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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.
Connectivity failure triggering a switchover can include, for example, one of the following:
If the active device runs an earlier version (e.g., 7.0) than the redundant device
(e.g., 7.2), the redundant device is downgraded to the same version as the active
device (e.g., 7.0).
You can delay the transition from HA non-operational state, which occurs during
HA synchronization between the active and redundant device, to HA operational
state. This is configured by the [HAOperationalStateDelayInSec] parameter. This
may be useful, for example, to delay HA switchover when using switches with
spanning tree protocol (STP) that take a long time until their ports (to which the
redundant device is connected) is ready.
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Mediant 4000 SBC | User's Manual