Avaya 9608 Administrator's Manual page 151

Transition automatically to lower priority controllers whenever a high priority controller fails
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or becomes unreachable (automatic failover),
Transition from lower priority controllers to a high priority controller (failback) either
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automatically or as a result of explicit administrator activity,
Preserve active calls to the greatest extent possible in the event of a transition, and
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Preserve as many call and system features as possible when operating under failure
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conditions.
Be in a pushable state during transition, when the primary controller is lost and the
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deskphone is not connected to a secondary controller. Once the phone is registered on
secondary controller (AudioCodes, Cisco, etc.) and regardless if the phone is active on a
call, the phone is in a pushable state, just as if were connected to primary server. The
phone is always in a pushable for state for all normal or barge-in Top Line. Display, Audio
Receive/Transmit, or phonexml pushes for all transition conditions.
In general, the phone does not attempt to preserve SIP transactions in progress when a
controller failure is detected, and some mid-call features like conferencing can fail. However, in
some scenarios the same transaction may succeed if re-attempted once the transition to a new
controller has been completed.
The deskphone always registers to a configured controller with the credentials (username/
password) of the user who is currently logged-in, even if the deskphone transitions from one
controller to another.
As described in the respective deskphone user guide, certain features may not be available and
functionality may be limited or work differently during any stage of failover, "limbo," transition, or
failback. Calls can still be placed and received, and other deskphone functions remain active.
The following apply when a deskphone is in failover mode:
If the user is active on a call, a failover icon displays when failing over to a non-AST
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controller and messages like "Link recovery." "Limited phone service." and "Calls may be
lost." inform the user of a failover situation. The message "Limited phone service" also
displays during failover transition from one Session Manager server to another when the
subscriptions have not yet been moved successfully to the secondary SM. The only user
options are to navigate to the Phone screen by pressing the OK softkey or the Phone
button or hang up the call.
When failing over to a third party secondary controller, an Acquiring Services screen
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displays during failover and failback transitions to an active controller and a Call
Preservation message window may display for deskphones running SIP software Release
2.6 or greater. Most other screens and applications except for Craft screens are
unavailable until an active controller is found. However, the user can navigate to the
Contacts, Call Forward feature, Avaya (A) Menu or Call Log applications (if administered).
Pressing the Phone button causes the user to be redirected to the Home Screen (if one is
administered).
If a call is active when failover occurs, that call will remain active. The user cannot initiate
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new calls while the phone transitions to the alternate server.
Controller Determination and Survivability Activity
Issue 1 May 2011 151