DHCP Relay Description
DHCP Relay must be enabled globally and per VLAN.
In Layer 2 mode, the switch can relay DHCP messages received from a VLAN to one or more configured DHCP servers. When the DHCP Relay receives a DHCP message from a station, it adds option 82 to the frame to retain the input VLAN and input port. When a response is received from a DHCP server, the feature removes option 82 from the frame, and uses the option to determine where the station is connected. In this mode, the DHCP Relay discards all DHCP frames received from stations already containing option 82.
In Layer 3 mode, the switch can relay DHCP messages received from its IPv4 interfaces to one or more configured DHCP servers. The switch puts the IPv4 address into the message giaddr before relaying it to the servers. It uses the switch IPv4 address of the interface where the message is received. The switch uses the giaddr from the response to determine how to forward the response back to the DHCP client.