Configuration guide of DHCP Options on Omada Switch

Contents

This article introduces common DHCP Options, including their features, functions, and how to customize DHCP Options for switches on the Omada Controller.

Requirements

Omada Smart/ L2+/L3 switches
Omada Controller (Software Controller / Hardware Controller / Cloud-Based Controller, V5.9 and above)

Introduction

DHCP Options are used to carry additional information and parameters in DHCP messages. Every option shares a common base format, as showed in the following figure. All values in options are represented in network byte order. The use of DHCP Options helps simplify network management with less manual configuration and improves network flexibility and extensibility.

The value of DHCP Options field ranges from 1 to 255. The following table lists some common DHCP Options and their features.

Option No.	Explanation
2	Option 2 is used by clients to configure the time offset. The time offset field specifies the offset of the client’s subset in seconds from Coordinated Universal Time (UTC).
42	Option 42 is used by clients to configure the NTP server address.
43	Option 43 is used by clients and servers to exchange the vendor-specific information.
44	Option 44 is used by clients to configure the NetBIOS over TCP/IP name server.
60	Option 60 is used by clients to optionally identify the vendor type and configuration of a DHCP client. Mostly it is used in the scenario where APs apply for different IP addresses from different servers according to the needs.
66	Option 66 specifies the TFTP server information and supports a single TFTP server IP address.
67	Option 67 tells the client a path to a file from a TFTP server (Option 66) that will be retrieved and used to boot. That file needs to be a basic boot loader that conducts any other required work.
138	Option 138 is used to discover devices by the Omada Controller.

For the definition and usage of other DHCP Options, refer to RFC 2132: DHCP Options and BOOTP Vendor Extensions (rfc-editor.org).

In such scenarios as school campus network or enterprise network, a core switch may act as a DHCP server that will automatically assign IP addresses and other network information to the devices connected to the switch.

The following topology will be used as an example to show how to use and configure DHCP Options on the Omada Controller for the switch. In the network topology, Port 1/0/28 is an uplink port connected to the internet; Port 1/0/24 is connected to the Omada Controller whose IP address can be set in the Management VLAN. Here, the IP address of the Controller is assigned to the network segment of 192.168.0.0/24. For more information on Management VLAN settings, refer to How to configure Management VLAN in Omada SDN Controller (4.4.4 or above) | TP-Link. Port 1/0/1 is connected to a PC client with the IP address of 192.168.100.0/24. This IP address is assigned by the DHCP Server configured on the switch.

Configuration

Step 1. Create new VLAN on the Controller. Go to Settings > Wired Networks > LAN and click Create NEW LAN.

Specify the Name, Purpose, VLAN and other parameters for the newly created VLAN. Click Save to save the configuration.

Step 2. Configure the existing DHCP Options on the Controller. Go to Devices > Gateway/Switches and click the target switch to reveal the Properties window.

In the Properties window of the switch, go to Config > VLAN Interface and enable VLAN_100. Click the Edit icon to edit the interface of VLAN_100.

Select Static for IP Address Mode and specify the IP Address and Subnet Mask. Select DHCP Server for DHCP Mode and enter the required parameters such as DHCP Range and Primary DNS. In the optional field of DHCP Option 138, you can specify the Controller’s IP address according to your own needs. Click Apply to save the configuration.

After completing the settings above, you have successfully configured the DHCP Option 1 (Subnet Mask), Option 3 (Router), Option 6 (Domain Name Server), Option 51 (Lease Time), and Option 138.

Note: The VLAN interface requires an IP address to use the DHCP Server. Setting the IP Address Mode of VLAN_100 as Static can allow VLAN_100 to use the DHCP Server to assign IP addresses.

Step 3. Configure custom DHCP Options (optional). To configure other DHCP Options, use CLI commands. Go to Settings > CLI Configuration > Device CLI and click Create New Device CLI Profile.

Configure DHCP Option 121 through the CLI commands below. Option 121 is a Classless Static Route option that defines a series of static routes which can be assigned to the client and added to the client’s routing table. The value of Option 121 is an IP address in hexadecimal format.

Here, the Destination Address is 172.16.0.0/16 (represented in hexadecimal as 10AC10), and the Next Hop is 192.168.100.1 (represented in hexadecimal as C0A86401).

ip dhcp server pool p_100

option 121 HEX 10AC10C0A86401

Specify Name and CLI, and click Next.

Select the switch to be configured and click Confirm.

Then click Save to save the configuration.

Click Apply to activate the CLI configuration.

Step 4. Apply the configuration to the target port. Go to Devices > Gateway/Switches and click the target switch to reveal the Properties window. Go to Ports and click Edit to configure the target port.

Select the CLI profile created. Here select P_100. Click Apply to apply the configuration to the port.

Verification

Run the command prompt (cmd) on a PC and enter the commands ipconfig and route print respectively. It shows that the client have obtained the IP address and other information and a static route has been configured.

Conclusion

You have successfully configured DHCP Option on the Omada Controller.

Get to know more details of each function and configuration please go to Download Center to download the manual of your product.