The persistence feature allows lease information about DHCP servers to be kept across reboots. This information can include data such as the IP address, MAC binding information, and lease length information.
The system performs the following tasks to make data persistent. In systems with only one CSM, only task 1 applies. In systems with dual CSMs, both tasks apply.
When a DHCP ACK is received from a DHCP server, the entry information is written to the active CSM compact flash. If persistence fails completely (bad cflash), a trap is generated indicating that persistence can no longer be guaranteed.
DHCP message information is sent to the standby CSM, and the DHCP information is also written to the compact flash. If persistence fails on the standby CSM also, a trap is generated.