Setting up a gateway with Comcast, non-static

[ChrisChivers]

I have a client that refuses to spend the $20 for a static IP through Comcast and they want me to install a fortigate 60f in their home office. I’ve set the Comcast XB-7T gateway device to bridge mode, but the Fortigate is not pulling an IP address when the Wan interface is set to DHCP. When the gateway device is not in bridge mode, I get a private IP address (10.0.0.x) on the wan port of the fortigate without a problem, but this puts devices behind the firewall in a double nat situation, which is causing massive network slowdowns (800mb drops to 20mb connection). He also wants to be able to use VPN to access his network from outside (planning on using a DDNS for when the IP changes) as needed, so I need to get this working fit them. I am open to suggestions on what I am doing wrong. I never have these problems when the client gets a static IP. Thank you in advance. - Chris

[lobstercreed]

Have you tried a packet capture to see what's happening in bridge mode?  I'd be going at this from the Comcast angle as their device should be able to hand DHCP downstream, but might need a reboot.  Is it PPPoE maybe?

[ChrisChivers]

They insist it is not PPPoE. Sniffing traffic is a good idea. I'll try that next.