Reference Guide

DHCP 439
Overview
The device can act as a DHCP Relay agent that listens for DHCP messages,
and relays them between DHCP servers and clients, which reside in different
VLANs or IP subnets.
This functionality is intended to be used when the client ingress VLAN is
different than the VLAN on which DHCP servers are connected.
The switch can relay DHCP messages received from its IPv4 interfaces to one
or more configured DHCP servers. It uses the switch’s IPv4 address of the
interface where the message is received. The switch uses the address from the
response to determine how to forward the response back to the DHCP client.
DHCP Relay must be enabled globally and per VLAN.
Limitations
The following limitations exist for DHCP Relay:
• It is not supported on IPv6.
• It is not relayed to servers on the client’s VLAN.
• Packets that have option-82 information, added by other devices, are
discarded.
• It does not support Option 82 on non-VLAN interfaces.
• It can be enabled only on a VLAN/Port/LAG that has an IP address defined
on it.
Option 82
The relay agent information option (Option 82) in the DHCP protocol
enables a DHCP relay agent to send additional client information when
requesting an IP address. Option 82 specifies the relaying switch's MAC
address, the port identifier, and the VLAN that forwarded the packet.
Both DHCP snooping and DHCP relay can insert option 82 into traversing
packets.
DHCP snooping with option 82 insertion provides transparent Layer 2 relay
agent functionality when the DHCP server is on the same VLAN as the
clients.