Dell PowerConnect M6348 Configuration Manual page 45

snooping removes bindings in response to DECLINE, RELEASE, and NACK messages. DHCP
Snooping application ignores the ACK messages as reply to the DHCP Inform messages received on
trusted ports. The administrator can also enter static bindings into the binding database.
The DHCP binding database resides on a configured external server or locally in flash depending upon
the user configuration. 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 bindings 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, the entry is removed. If the system time
is not consistent across reboots, 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 will go to the tentative binding.
Figure 3-3. DHCP Binding
DISCOVER,
Tentative
binding
The DHCP snooping component does not forward server messages since they are forwarded in hardware.
DHCP snooping forwards valid DHCP client messages received on un-trusted interfaces to all trusted
interfaces within the VLAN.
The binding's database includes the following information for each entry:
• Client MAC address
• Client IP address
• Time when client lease expires
• Client VLAN ID
• Client port
binding
REQUEST
DECLINE,
NACK
DISCOVER
No
RELEASE,
NACK
ACK
Complete
binding
Switching Configuration
45