Chapter 4 FortiOS Carrier : Troubleshooting : GTP packets are not moving along your network : Check the logs and adjust their settings if required
  
Check the logs and adjust their settings if required
During normal operation, the log settings will show any problems on the network but may not provide the level of details required to fully troubleshoot the problem. The reason for this is that the level of detail required for troubleshooting would quickly overwhelm the daily logs without any real benefit.
GTP related events in the event log will have message IDs in the range 41216 to 41222. For more information on GTP log messages, see the Log Message Reference. For more information on logging in general, see the Logging and Reporting handbook chapter.
Once there is a problem to troubleshoot, check the logs to trace the traffic patterns and narrow down the possible sources of the problem. There may be enough detail for you to locate and fix the problem without changing the log settings.
 
Remember to set any changes you made to the log settings back to their original values when you are done troubleshooting. Otherwise, the amount of detail will overwhelm your logging.
However, if more detail is required you can change settings such as:
Lower the Log Frequency number in GTP Profiles so fewer or no log messages are dropped. This will allow a more accurate picture of everything happening on the network, where you may have had only a partial picture before.
Ensure all the GTP log events are enabled to provide you with a complete picture.
Ensure that all relevant event types are enabled under Log & Report > Log Config > Event Log.
For more information on GTP related logging, see “Logging events on the Carrier-enabled FortiGate unit”. See the log and report chapters of Logging and Reporting Guide and FortiOS CLI Reference.
General information to look for in the logs includes:
Are all packets having problems or just certain types?
Are all devices on the network having problem, or just certain devices?
Is it just GTP traffic that is having problems or are all types of traffic having the same problem?