Linksys LGS308 User Manual page 137

• DHCP Insertion - Add Option 82 information to packets that do not have foreign Option
82 information.
• DHCP Passthrough - Forward or reject DHCP packets that contain Option 82 information
from untrusted ports. On trusted ports, DHCP packets containing Option 82 information
are always forwarded.
DHCP Snooping Binding Database
DHCP Snooping builds a database (known as the DHCP Snooping Binding database) derived from
information taken from DHCP packets entering the device through trusted ports.
The DHCP Snooping Binding database contains the following data: input port, input VLAN, MAC
address of the client, and IP address of the client if it exists.
The DHCP Snooping Binding database is also used by IP Source Guard and Dynamic ARP
Inspection features to determine legitimate packet sources.
DHCP Trusted Ports
Ports can be either DHCP trusted or untrusted. By default, all ports are untrusted. To create a
port as trusted, use the DHCP Snooping Trusted Interface page. Packets from these ports are
automatically forwarded. Packets from trusted ports are used to create the Binding database and
are handled as described below.
If DHCP Snooping is not enabled, all ports are trusted by default.
How the DHCP Snooping Binding Database is Built
The following describes how the device handles DHCP packets when both the DHCP client and
DHCP server are trusted. The DHCP Snooping Binding database is built in this process.
DHCP Trusted Packet Handling
1. Device sends DHCPDISCOVER to request an IP address or DHCPREQUEST to accept an
IP address and lease.
2. Device snoops packet and adds the IP-MAC information to the DHCP Snooping Binding
database.
3. Device forwards DHCPDISCOVER or DHCPREQUEST packets.
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