Fortigates incompatibility with Telenet DOCSIS 3.1 E-ROUTER
If you are using a Fortigate with the Belgian Cable ISP Telenet, you should know that there is a known incompatibility between Fortigates and the new Telenet DOCSIS 3.1 E-ROUTER. This is the kind of router that they install for all new clients. For now, it is possible to request Telenet to install a DOCSIS 3.0 modem and that solves the issue, but going forward this could become a big problem, especially if they require all their clients to switch to DOCSIS3.1.
The problem seems to be that Fortigates don't support Unicast DHCP.
As far as I can tell the incompatibility is known to Telenet, but I don't know if Fortinet knows or not. I have opened a TAC case to find out.