Periodic I-U message output can be paced to avoid congestion at the logging server. Pacing is controlled by the rate-limit option of the periodic-update command. As an example, consider the following hypothetical case:
1 million NAT subscribers came up within 1hour (16,666 NAT-subs per minute).
On average, each NAT subscriber allocates two port blocks.
This means that 2 million logs are sent to the logging server.
If the rate-limit value is set to 100 (messages per second), on average it would take over 5 hours to send all those messages at the given rate.
In this case, it would be prudent to set the interval value to at least 6 hours, or increase the rate-limit value so there is no time overlap between the old and new logs.
In the case of an MS-ISA switchover or a NAT multi-chassis redundancy switchover, there is a chance that a large number of subscribers become active at approximately the same time on the newly active MS-ISA (or chassis). This causes a large number of logs to be sent in a relatively short amount of time, which may overwhelm the logging server. The rate-limit parameter is designed to help in such situations.