How Is The Dhcp Snooping Bindings Database Populated - Dell PowerConnect 8024 User Configuration Manual

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How Is the DHCP Snooping Bindings Database Populated?

The DHCP snooping application uses DHCP messages to build and maintain

the binding's database. DHCP snooping creates a tentative binding from

DHCP DISCOVER and 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.

When a switch learns of new bindings or 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, that entry is

removed. Make sure the system time is consistent across the reboots.

Otherwise, the snooping entries will not expire properly. If a host sends a

DHCP release while the switch is rebooting, when the switch receives the

DHCP discovery or request, the client's binding goes to the tentative binding

as shown in Figure 27-1.

Figure 27-1. DHCP Binding

The binding database includes data for clients only on untrusted ports.

Snooping and Inspecting Traffic

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