454 Configuring Switching InformationDHCP Snooping Static Bindings ConfigurationUse the DHCP Snooping Static Bindings Configuration page to add static DHCP bindings to thebinding database.The DHCP snooping application uses DHCP messages to build and maintain the binding’s database.The binding’s database only includes data for clients on untrusted ports. DHCP snooping creates atentative binding from DHCP DISCOVER and REQUEST messages. Tentative bindings tie a client to aport (the port where the DHCP client message was received). Tentative bindings are completed whenDHCP snooping learns the client’s IP address from a DHCP ACK message on a trusted port. DHCPsnooping removes bindings in response to DECLINE, RELEASE, and NACK messages. The DHCPsnooping application ignores the ACK messages as a reply to the DHCP Inform messages received ontrusted ports. You can also enter static bindings into the binding database.The DHCP binding database is persisted on a configured external server or locally in flash, depending onthe user configuration. A row-wise checksum is placed in the text file that is going to be stored in theremote configured server. On reloading, the switch reads the configured binding file to build the DHCPsnooping database. When the switch starts and the calculated checksum value equals the storedchecksum, the switch reads entries from the binding file and populates the binding database. Achecksum failure or a connection problem to the external configured server will cause the switch to loosethe bindings and will cause a host’s data loss if DAI is enabled.When a switch learns of new bindings or when it loses bindings, the switch immediately updates theentries in the database. The switch also updates the entries in the binding file. The frequency at whichthe file is updated is based on a configurable delay, and the updates are batched.If the absolute lease time of the snooping database entry expires, then that entry will be removed. Youshould take care of the system time to be consistent across the reboots. Otherwise, the snooping entrieswill not expire properly. If a host sends a DHCP release while the switch is rebooting then, when theswitch receives the DHCP discovery or request, the client’s binding will go to the tentative binding asshown in the following figure.Table 7-68. DHCP Snooping Persistent Database CommandsCLI Command Descriptionip dhcp snooping database Configures the persistent location of the DHCP snooping database.ip dhcp snooping database write-delay Configures the interval in seconds at which the DHCP Snoopingdatabase will be stored in persistent storage.show ip dhcp snooping database Displays the DHCP snooping configuration related to the databasepersistence.
