Question: When an MME supporting CS service restoration via an alternative VLR detects a serving VLR failure upon receiving a UE's periodic tracking area update request, what are the possible MME actions based on network configuration and operator policy?
Option 1: Immediately select an alternative VLR and perform a location update for non-EPS services.
Option 2: Perform a network-initiated detach with 'IMSI detach' immediately after completing the periodic tracking area update, then select an alternative VLR during the next combined tracking area update.
Option 3: Either (a) select an alternative VLR and immediately perform the location update for non-EPS services, or (b) perform a network-initiated detach with 'IMSI detach' after completing the periodic update, then select an alternative VLR during the subsequent combined update, based on network configuration and operator policy.
Option 4: Request the UE to re-attach for non-EPS services without selecting an alternative VLR.
Explanation: This option correctly reflects the MME's possible actions in response to a VLR failure during a periodic tracking area update. It acknowledges the dependency on network configuration and operator policy, offering two potential courses of action: immediate alternative VLR selection and location update, or a staged approach involving initial detach followed by alternative VLR selection during a subsequent combined update.