Bridge mode means that the "modem" doesn't do any Level 3 processing on the traffic and passes it through, sort of like an Ethernet switch, e.g. you wouldn't see a HOP in traceroute. With the AT&T Gateway you cannot achieve a true bridge mode. The best you can do is:
For a static public subnet: the Cascaded Router (if you want to use a router to manage the public subnet)
For the dynamic public subnet: IP Passthrough (or DMZplus on the Gateways that call the feature by that name).
When configured for IP Passthrough (Passthrough Mode) the AT&T provided gateway shares its Dynamic WAN IP address with a single device on the LAN.
would recommend adding a bridging router that would be connected to both networks. You can use pretty much any router with at least two Ethernet ports and that can run aftermarket firmware (like Tomato, OpenWRT, DD-WRT, and so on).
All you have to do in the router that connects the two networks it this:
Configure each of two Ethernet ports into different VLANs.
Assign each VLAN an IP address in one of your two networks.
Connect each port to the network that port's VLAN has an IP address in.
Make sure the router is configured to do routing. (This is the default in most distributions.)
Then, to make it work, you'll have to log into each of your two existing routers and add a route. For example, if the bridging router is 192.168.1.2 and 192.168.2.2, you'll need to add these two routes:
In the 192.168.1.x network, a route to 192.168.2.0/24 with a gateway of 192.168.1.2
In the 192.168.2.x network, a route to 192.168.1.0/24 with a gateway of 192.168.2.2
Note that computers in the two networks will be in different broadcast domains, so they won't easily discover each other. Depending on what tools you plan to use, there are various ways around this. For example, if you use Windows networking a lot, you can use a program (like nmbd) that synchronizes browse lists across the two networks.
I have 1Gig Fiber connection. I have Eth0 as WAN. Eth1 is LAN connected to a Asus XG-U2008 unmanaged switch where all the other wired devices are connected to. ( I do not have that many devices, 9 wired, 7 wifi)
My Asus RT-AC86U is now just a AC WiFi AP. (running Merlin Firmware)
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