Question: What triggers the increment of the 'Failed intra 3G-SGSN SRNS Relocations, due to external reasons' performance measurement counter in a UMTS network?
Option 1: Successful handover between two RNCs under the same SGSN
Option 2: Receipt of 'Relocation Failure' message from the Target RNC due to an external reason
Option 3: Internal SGSN software failure leading to SRNS relocation cancellation
Option 4: Expiration of a timer while waiting for a response from the Core Network during an inter-SGSN relocation attempt
Explanation: The correct answer directly relates to one of the specified external reasons for failed intra 3G-SGSN SRNS relocations. The 'Relocation Failure' message from the TRNC indicates an issue outside the current SGSN's control, fitting the criteria for incrementing this performance measurement counter, relevant for monitoring and optimizing handover success rates between different radio network subsystems under the same serving node.