D-Link DWS-4026 Product Manual - Page 179

Software User Manual 12/10/09 D-Link Unified Access System REQUEST messages. Tentative bindings tie a client to a port (the port where the DHCP client message was received). Tentative bindings are completed when DHCP snooping learns the client's IP address from a DHCP ACK message on a trusted port. DHCP snooping removes bindings in response to DECLINE, RELEASE, and NACK messages. The DHCP snooping application ignores the ACK messages as a reply to the DHCP Inform messages received on trusted 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 on the user configuration. A row wise checksum is placed in the text file that is going to be stored in the remote configured server. On reloading, the switch reads the configured binding file to build the DHCP snooping database. When the switch starts and the calculated checksum value equals the stored checksum, the switch reads entries from the binding file and populates the binding database. A checksum failure or a connection problem to the external configured server will cause the switch to loose the bindings and will cause a host's data loss if IP Source Guard (IPSG) and/or DAI is enabled. When a switch learns of new bindings or when it loses bindings, the switch immediately updates the entries in the database. The switch also updates the entries in the binding file. The frequency at which the 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. You should take care of the system time to be consistent across the reboots. Otherwise, the snooping entries will not expire properly. If a host sends a DHCP release while the switch is rebooting then, when the switch receives the DHCP discovery or request, the client's binding will go to the tentative binding as shown in Figure 104 on page 179. No Binding DeclinDei,scNoAvCerKRequest Release, NACK Tentative Binding Discover ACK Complete Binding Figure 104: States of Client Binding To access the DHCP Snooping Static Binding Configuration page, click LAN > L2 Features > DHCP Snooping > Binding Configuration in the navigation tree. Document 34CSFP6XXUWS-SWUM100-D7 Configuring DHCP Snooping Page 179