To retain the bindings across reloads, you must use the DHCP snooping database agent. Without this agent, the bindings established by DHCP snooping are lost upon reload, and connectivity is lost as well.
The database agent stores the bindings in a file at a configured location. Upon reload, the switch reads the file to build the database for the bindings. The switch keeps the file current by writing to the file as the database changes.
The format of the file that contains the bindings is as follows:
Each entry in the file is tagged with a checksum that is used to validate the entries whenever the file is read. The <initial-checksum> entry on the first line helps distinguish entries associated with the latest write from entries that are associated with a previous write.
This is a sample bindings file:
126.96.36.199 512 0001.0001.0005 3EBE2881 Gi1/1 e5e1e733
188.8.131.52 512 0001.0001.0002 3EBE2881 Gi1/1 4b3486ec
184.108.40.206 1536 0001.0001.0004 3EBE2881 Gi1/1 f0e02872
220.127.116.11 1024 0001.0001.0003 3EBE2881 Gi1/1 ac41adf9
18.104.22.168 1 0001.0001.0001 3EBE2881 Gi1/1 34b3273e
Each entry holds an IP address, VLAN, MAC address, lease time (in hex), and the interface associated with a binding. At the end of each entry is a checksum that is based on all the bytes from the start of the file through all the bytes associated with the entry. Each entry consists of 72 bytes of data, followed by a space, followed by a checksum.
Upon bootup, when the calculated checksum equals the stored checksum, the switch reads entries from the file and adds the bindings to the DHCP snooping database. If the calculated checksum does not equal the stored checksum, the entry read from the file is ignored and so are all the entries following the failed entry. The switch also ignores all those entries from the file whose lease time has expired. (This is possible because the lease time might indicate an expired time.) An entry from the file is also ignored if the interface referred to in the entry no longer exists on the system, or if it is a device port or a DHCP snooping-trusted interface.
When the switch learns of new bindings or when it loses some bindings, the switch writes the modified set of entries from the snooping database to the file. The writes are performed with a configurable delay to batch as many changes as possible before the actual write happens. Associated with each transfer is a timeout after which a transfer is aborted if it is not completed. These timers are referred to as the write delay and abort timeout.