DHCP Server behind DHCP Relay - requires route to subnet?

Hello everyone

we are in the process of setting up multiple vlans at a customer and are wondering about some setting concerning DHCP relay.

We are using a switch to relay DHCP requests from different vlans to our sophos SG firewall which has DHCP servers for the scopes and knows they are behind a relay. Between the firewall and the switch there is a transfer network just for internet and DHCP.

Now, we can see the DHCP dicovery coming from the switch to the firewall in a packet trace but the firewall is unable to answer it.

If we create the following static route on the firewall everything works fine:

Destination: Subnet in which we want DHCP | Gateway: Interface of DHCP Relay in VLAN between DHCP Relay and Server

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Is this routing rule required and should we create a route for all subnets that will receive an IP address via DHCP, oder is there something wrong with our configuration?

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Thanks in advance and best regards!

Florian