The Monitoring and Reports part of ACS 5.x contains information about the incoming authentication requests (TACACS+ and RADIUS) also information about the health of each ACS node in the deployment. The Monitoring and Report Dashboard gives us a quick and easy way to get to both authentication and health information.
Out of the box, the Monitoring and Reporting dashboard is populated with the “Top 5 Alarms” and “My Favourite Reports” applications (more applications can be added by clicking on the “Configure” button in the top right-hand corner of the dashboard). One of the more common alarms that appears in the “Top 5 Alarms” list is the “delete 20 000 sessions” alarm. At first glance the alarm name may give the impression that the ACS is deleting key information or perhaps that it’s unexpectedly kicking off 20 000 users. So, what does this alarm really mean?
This alert is informational and is generated because the ACS View keeps track of authentication sessions.
The ACS View maintains all the sessions (RADIUS/TACACS Authentication/Authorization/Accounting). It can keep only 250,000 sessions at a time. Whenever it crosses 250k, it will try to delete 20k sessions, and will send an alarm (the one that you see on the ACS Dashboard).
The ACS normally keeps track of the session authentications by following accounting records ACCOUNT_START and ACCOUNT_STOP. However, if ACS View does not get ACCOUNT_STOP records, the number of sessions will not be decreased. As a result, any active sessions for which the ACS View does not receive an ACCOUNT_STOP will remain and then expire after two days.