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Preservation of RADIUS Accounting Information During an Accounting Server Outage

If the router loses contact with the RADIUS accounting server, as represented in Figure 1, whether due to a server outage or a problem in the network connecting to the server, you can lose all the billing information that would have been received by the server. RADIUS accounting backup preserves the accounting data that accumulates during the outage. If you have not configured RADIUS accounting backup, the accounting data is lost for the duration of the outage from the time when the router has exhausted its attempts to resume contact with the RADIUS server. The configurable retry value determines the number of times the router attempts to contact the server.

Figure 1: Topology with Loss of Access to Accounting Server

1. The router fails to receive accounting acknowledgments from the server.
2. The router immediately attempts to contact the accounting server and marks the server as offline if the router does not receive an acknowledgment before exhausting the number of retries.
3. The router next attempts to contact in turn each additional accounting server configured in the RADIUS profile.

   If a server is reached, then the router resumes sending accounting requests to this server.

4. If none of the servers responds or if no other servers are in the profile, the router declares a timeout and begins backing up the accounting data. It withholds all accounting stop messages and does not forward new accounting requests to the server.
5. During the outage, the router sends a single pending accounting stop message to the servers at periodic intervals.
6. If one of the servers acknowledges receipt, then the router sends all the pending stop messages to that server in batches at the same interval until all the stored stop messages have been sent. However, any new accounting requests are sent immediately rather being held and sent periodically.

The router replays accounting stop messages to the server in the correct order because it preserves both the temporal order among subscribers and the causal order between service and session stop requests for each subscriber. Only accounting stop messages are backed up, because they include the start time and duration of sessions and all the accounting statistics. This makes it unnecessary to withhold the accounting start messages, which eventually time out. Interim updates are not backed up and time out as well; if the session remains active, then the next interim update after the server connection is restored provides the interim accounting information.

You can configure the number of accounting stop messages that the router can queue pending restoration of contact with the accounting server. To preserve current accounting data in preference to collecting new accounting data, subscriber logins fail as soon as the maximum number of messages has been withheld. Subscriber logins resume immediately when the pending queue drops below the queue limit.

Note: Service accounting stop messages are withheld for a maximum of ten services per subscriber. If a subscriber attempts to activate an eleventh service while that accounting server is offline, the activation fails.

The router can hold the pending accounting messages for up to 24 hours. When the configurable maximum holding period passes, all accounting stop messages still in the pending queue are flushed, even if the accounting server has come back online. A consequence of this is that subscriber logins resume immediately if they were failing because the maximum pending limit had been reached.

• If you remove the last accounting server from the access profile, because then there is no place to send the messages.
• If you remove the accounting backup configuration.

• If the router receives an acknowledgment of receipt, then it marks the server as online and begins replaying all remaining pending stop messages in batches.
• If the router does not receive the acknowledgment, then it resumes sending a single pending accounting stop message at the periodic interval.

When a subscriber logs out while the accounting server is offline, the accounting stop requests for the subscriber and the session are queued and replayed to the server when it comes online. In this case, the subscriber session and service session information is retained, so that the router can send a correct accounting request when the server comes back online.

In the event of a graceful Routing Engine switchover while the accounting server is offline, the pending stop messages can be replayed from the active Routing Engine when the server is online again.

Note: When RADIUS accounting backup is configured, you must use different servers for RADIUS authentication and accounting. Subscriber authentication fails when the same server is configured for both authentication and accounting. If the RADIUS server acts on behalf of other back-end RADIUS accounting or authentication servers and forwards requests to them, subscribers can be authenticated but accounting requests are not sent out.