USG FLEX H Series - DHCP Relay and DHCP Options

Zyxel_Richard Posts: 249  Zyxel Employee
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edited May 17 in Networking

USG FLEX H Series - DHCP Relay and DHCP Options

DHCP Relay Agent


In network environments, it’s sometimes necessary for DHCP clients to receive IP addresses from a DHCP server located on a different subnet. This is where the DHCP relay agent comes into play. The DHCP relay agent forwards DHCP requests from clients in one subnet to a DHCP server in another subnet, ensuring that clients can still receive their necessary IP configurations.

Traditional Scenario

Traditionally, DHCP clients broadcast a DHCP Discover message to find a DHCP server. However, broadcast messages do not travel across different subnets. In a network without a DHCP relay, clients on different subnets from the DHCP server would not be able to obtain an IP address.

Enhanced DHCP Relay in USG FLEX H Series

In the previous firmware versions (1.10), the DHCP relay function required the DHCP server to be directly connected to the firewall. With the 1.20 firmware update, the USG FLEX H Series now supports relaying DHCP packets even if the DHCP server is multiple hops away from the firewall.

New Scenario

  • Client Subnet:
  • DHCP Server Subnet:
  • Intermediate Devices: The firewall can relay DHCP requests across multiple intermediate devices (hops).

Configuration Steps

  • Navigate to DHCP Relay Settings:
  • Go to Network > Interface > DHCP > DHCP Relay.
  • Create a DHCP Relay Profile:
    • Select the interface facing the DHCP clients.
    • Specify the upstream interface (the interface that connects to the network where the DHCP server resides).
  • Enter the DHCP Server IP:
    • Provide the IP address of the DHCP server.
  • Save and Apply:
    • Ensure the settings are saved and applied for the relay agent to function correctly.

DHCP Extended Options

DHCP extended options allow network administrators to provide additional configuration parameters to DHCP clients. These options can be crucial for specific network setups, such as configuring DNS servers, setting up network time protocols, or defining certain policy rules.

Configuring DHCP Extended Options

  • Navigate to DHCP Server Settings:
  • Go to Network > Interface > DHCP > DHCP Server.
  • Select Interface:
    • Choose the interface where the DHCP server is configured.
  • Configure Extended Options:
    • Under the DHCP server settings, look for Extended Options.
    • Add the necessary DHCP options by specifying the option code and corresponding values.

Common DHCP Extended Options

  • Option 3: Default Gateway
  • Option 6: DNS Servers
  • Option 15: Domain Name
  • Option 42: Network Time Protocol (NTP) Servers


The enhanced DHCP relay and extended options in the USG FLEX H Series provide greater flexibility and control in managing network configurations. By enabling DHCP relay across multiple subnets and adding extended DHCP options, network administrators can ensure seamless connectivity and configuration for all network devices.