Troubleshooting Tip: Unable to create additional DHCP servers associated to one interface

Description

This article describes an issue where it is not possible to create more than one DHCP server on the same interface in FortiOS v8.0.0, even when IPAM is disabled.

Scope

FortiOS v8.0.0.

Solution

In some deployments, administrators may need to configure multiple DHCP servers on the same interface. For example, a FortiGate may provide DHCP services for multiple subnets while another device on the network acts as a DHCP relay.

The following example shows a FortiGate configuration with a single DHCP server associated with port3.

config system dhcp server
    edit 1
        set dns-service default
        set default-gateway 192.168.10.253
        set netmask 255.255.255.0
        set interface "port3"
            config ip-range
                edit 1
                    set start-ip 192.168.10.100
                    set end-ip 192.168.10.199
                next
            end
        set timezone-option default
    next
end

When trying to create an additional DHCP server, the FortiGate displays the following error:

'A DHCP server with dhcp-settings-from-fortiipam enabled must be the only server for the interface.'

config system dhcp server
    edit 2
        set dns-service default
        set default-gateway 192.168.20.253
        set netmask 255.255.255.0
        set interface "port3"
            config ip-range
                edit 1
                    set start-ip 192.168.20.100
                    set end-ip 192.168.20.199
                next
            end
        set timezone-option default
    next
end

FG-LAB (3) #     next
A DHCP server with dhcp-settings-from-fortiipam enabled must be the only server for the interface.
object check operator error, -7, discard the setting
Command fail. Return code -7

Although the error message suggests a FortiIPAM-related restriction, FortiIPAM is disabled on the device, as shown below. This indicates that the behavior is occurring independently of FortiIPAM configuration.

show system ipam | grep status
    set status disable

Note: 'dhcp-settings-from-fortiipam' is a per-DHCP-server setting and is separate from the global FortiIPAM service status. Due to this FortiOS v8.0.0 issue, the validation error may be displayed even when FortiIPAM and 'dhcp-settings-from-fortiipam' are disabled.

See config system dhcp server.

This behavior is addressed in FortiOS v8.0.1. Upgrading the device to this release or a later version resolves the problem.

Note: If a FortiGate running FortiOS v7.6.x that already has multiple DHCP servers associated with the same interface is upgraded to FortiOS v8.0.0, the additional DHCP servers are removed during the upgrade and cannot be recreated until the FortiGate is upgraded to FortiOS v8.0.1 or later.