This article explains how to configure legacy EX-Series switches as a Dynamic Host Configuration Protocol (DHCP) server, and lists the optional knobs that can be used during the configuration.

1. How can I configure my EX-Series switch as a DHCP server?

2. What are the steps to configure an EX Switch as a DHCP server and what options do I get while configuring the same?

Before configuring an EX-Series Switch as a DHCP server, you need to perform the following steps:  

Defining a DHCP pool

The DHCP pool name is identified by a subnet on the EX switch, which is configured on a physical layer 3 interface or on a VLAN interface (IRB). For example, if the switch has ports on VLAN 20, and interface VLAN 20 has IP address 20.20.20.1/24, then the pool to assign IP addresses for this subnet will be configured as set system services dhcp pool 20.20.20.0/24.

Alternatively, the IP address can also be assigned to a physical L3 interface. In either case, when the switch receives DHCP DISCOVER / REQUEST packets on either the layer 2 VLAN 20 ports, or the layer 3 port with an interface IP address, the switch knows to assign IP addresses from the pool identified by 20.20.20.0/24. This is how the switch can map DHCP requests to a specific pool.

You can create more pools for other subnets as needed.

Configuring the DHCP pool parameters

Reference the DHCP pool by name and configure these parameters.

• Address range for IP assignment

• Lease time in seconds

• DHCP Server Identifier

• Gateway / Router, WINS, DNS addresses

• DHCP Options if any, etc

Configure L2 ports with the correct VLAN assignment, or assign appropriate IP addresses to the layer 3 interface. Attach DHCP clients to these ports.

Example

[edit]
root@4200-48-2# show system services dhcp 
pool 20.20.20.0/24 {
    address-range low 20.20.20.50 high 20.20.20.100;
    default-lease-time 3600;
    name-server {
        20.20.20.1;
    }
    wins-server {
        20.20.20.1;
    }
    router {
        20.20.20.1;
    }
    server-identifier 20.20.20.1;
}

[edit]
root@4200-48-2# show system services dhcp | display set 
set system services dhcp pool 20.20.20.0/24 address-range low 20.20.20.50
set system services dhcp pool 20.20.20.0/24 address-range high 20.20.20.100
set system services dhcp pool 20.20.20.0/24 default-lease-time 3600
set system services dhcp pool 20.20.20.0/24 name-server 20.20.20.1
set system services dhcp pool 20.20.20.0/24 wins-server 20.20.20.1
set system services dhcp pool 20.20.20.0/24 router 20.20.20.1
set system services dhcp pool 20.20.20.0/24 server-identifier 20.20.20.1

[edit]
root@4200-48-2# show interfaces vlan.20   
family inet {
    address 20.20.20.1/24;
}

[edit]
root@4200-48-2# show interfaces ge-0/0/0 
unit 0 {
    family ethernet-switching {
        vlan {
            members 20;
        }
    }
} 

Attach a PC to ge-0/0/0 and a DHCP assignment takes place.

 Verifying the DHCP server configuration

root@4200-48-2> show system services dhcp pool 
Pool name           Low address      High address     Excluded addresses
20.20.20.0/24       20.20.20.50      20.20.20.100  

root@4200-48-2> show system services dhcp ?      
Possible completions:
  binding              Show DHCP client binding information
  conflict             Show DHCP address conflict
  global               Show DHCP global scope information
  pool                 Show DHCP address pool information
  statistics           Show DHCP statistics

2026-02-16: Minor, non-technical changes made for search optimization

2020-08-25: Article reviewed for accuracy; specific platforms (legacy EX) added; article relevant and valid