CHAPTER40 HA Overview
Mediant 800 Gateway & E-SBC | User's Manual
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.
● 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.
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.
As only the active device is cabled to the PSTN interfaces (e.g., E1/T1), you MUST enable the
Preempt mode and set the active device with the higher priority. When a switchover to the
redundant device occurs, only the IP calls are maintained; the PSTN calls are dropped (by sending
a SIP BYE message to the IP side). Thus, whenever the PSTN-connected device recovers from
failure, it becomes the active device again, providing connectivity to the PSTN.
When a switchover from active to redundant device occurs and the active failed unit
requires a return merchandise authorization (RMA), meaning that it will be out of service
for a long period, in order to maintain your PSTN calls, connect the same PSTN
equipment and in the same manner (same ports) to the redundant device. The
configuration between the devices is identical and thus, call routing process is
unaffected. When connected to the PSTN, new Gateway calls can be handled by the
newly active unit.
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:
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