Configuring a DHCP shared subnet

This information is also available in the FortiOS 7.4 Administration Guide: DHCP shared subnet

A FortiGate can act as a DHCP server and assign IP addresses from different subnets to clients on the same interface or VLAN based on the requests coming from the same DHCP relay agent. A FortiGate may have more than one server and pool associated with the relay agent, and it can assign IP addresses from the next server when the current one is exhausted. This way, the FortiGate can allocate IP addresses more efficiently and avoid wasting unused addresses in each subnet.

config system dhcp server
    edit <id>
        set shared-subnet {enable | disable}
        set relay-agent <ip_address>
    next
end

Example

In this example, there are two DHCP servers configured on the FortiGate. The first two clients (1 and 2) get their IP from the DHCP server 1. Once the DHCP server 1's IP pool is exhausted, subsequent clients (3 and 4) get their IP from DHCP server 2.

To configure a DHCP shared subnet:

1. Configure the DHCP servers:

   config system dhcp server
       edit 1
           set default-gateway 10.18.0.10
           set netmask 255.255.255.0
           set interface "p2_vl3819"
           config ip-range
               edit 1
                   set start-ip 10.18.0.110
                   set end-ip 10.18.0.111
               next
           end
           set shared-subnet enable
           set relay-agent 10.18.0.10
           set dns-server1 8.8.8.8
       next
       edit 2
           set default-gateway 10.18.1.130
           set netmask 255.255.255.128
           set interface "p2_vl3819"
           config ip-range
               edit 1
                   set start-ip 10.18.1.200
                   set end-ip 10.18.1.201
               next
           end
           set shared-subnet enable
           set relay-agent 10.18.0.10
           set dns-server1 8.8.8.8
       next
   end

2. Verify the DHCP lease list:

   # execute dhcp lease-list
   port6
     IP		MAC-Address	 Hostname    VCI    SSID    AP    SERVER-ID    Expiry
     10.18.0.110	00:50:56:02:92:11	                          1            Fri Jan 13 15:37:35 2023
     10.18.0.111	00:50:56:02:92:12	                          1            Fri Jan 13 15:37:38 2023
   
   Result: PASS

   Clients 1 and 2 get their IP from the DHCP server 1.

   When the IP pool is exhausted, the DHCP daemon assigns the IP from other pools that have the same relay agent.

3. Verify the DHCP lease list:

   # execute dhcp lease-list
   port6
     IP		MAC-Address	 Hostname    VCI    SSID    AP    SERVER-ID    Expiry
     10.18.0.110	00:50:56:02:92:11	                          1            Fri Jan 13 15:37:35 2023
     10.18.0.111	00:50:56:02:92:12	                          1            Fri Jan 13 15:37:38 2023
     10.18.1.200	00:50:56:02:92:13	                          2            Fri Jan 13 15:38:05 2023
     10.18.1.201	00:50:56:02:92:14	                          2            Fri Jan 13 15:38:06 2023

   Clients 3 and 4 get their IP from DHCP server 2, since the server 1 IP pool is exhausted.