Same problem here with an Arris tg3442de DOCSIS 3.1 cable router in bridge mode and a 60E.
If auto-negotiation is enabled, max downstream is ~110MBit/sec. If we place a gbit switch between the 60E and the Arris, we get 800-900Mbit/s (Cable speed is 1000Mbit/50Mbit)
As there is no possibility to set anything in the Arris router (Vodafone), we are now in the situation that the connection is shaky, as the Arris seem to rely on the link status on the LAN side for DHCP requests. If the FortiGate 60E Port is connected via switch and the router is restarted, 60E won't get a WAN IP from providers side, as first the 60E gets an internal IP of the Arris router's DHCP server. Then the Arris gets switched to bridge mode by the provider when cable sync finishes. As the 60E already got a DHCP address, it won't receive a new one from the provider, as the link is still up (to the switch). The providers DHCP server seem to register the device id and doesn't let the 60E get an IP address.
The only possible way we found to get an IP is to set the 60E WAN port to manual IP mode, restart the Arris and let it sync, then switch the 60E WAN port to DHCP. This way, the 60E receives an IP from the provider.
I do not know who is responsible for this bad performance when auto-negotiation is on, but a PC or MacBook works flawlessly on the Arris. 60E is on FW version 6.0.5.
Bottom line: The article https://kb.fortinet.com/kb/documentLink.do?externalID=13780
may be true in theory, but we got opposite results. We didn't expect auto-negotiation problems these days on gbit ethernet.
post edited by sym - 2019/06/21 01:56:05