First some details about my setup and what I've learned so far about bridge mode on the LB2120. The next posts will describe some issues I'm having and ask for help.

I'm using an LB210 in bridge mode with the WAN port connected to my LAN and the LAN port connected to a single raspberry pi. I'm using bridge mode because it allows me to reach the raspberry pi from my local network without setting up port forwarding rules as long as the wireline broadband connection is active (eg LB2120 not failed-over to mobile).

Based on observation, the LB2120's bridge mode doesn't behave like bridge mode on other routers. It does allow the single device connected to the LB2120 LAN port to be reached from devices on its WAN port without port forwarding rules, which is nice, but traffic between the WAN and LAN ports isn't actually bridged in a manner consistent with the IEEE's 802.1D bridging standard.

To make my setup work, first I assigned a static IP to the LB2120 for management (say 192.168.1.200), put the device into bridge mode, connected the raspberry pi to the LAN port and my local network to the WAN port. Next, I put a static DHCP entry into my main router with the mac address of the LB2120 and the IP address I want to assign to the raspberry pi (say 192.168.1.20). I added a second static DHCP entry in my router to reserve 192.168.1.200, but this entry has a junk mac address.

With this setup, the network on the WAN side of the LB2120 never sees the mac address of the raspberry pi (eg the device connected to the LAN port of the LB2120), only the mac address of the LB2120. The LB2120 appears to replace the mac layer as it moves packets between the WAN port and the LAN port.

As best as I can tell, at some point the LB2120 sends a DHCP request out of the WAN side to get an IP address for the device on the LAN side. I assume this request is made by the LB2120 in response to a DHCP request from the device on the LAN side. But the key is that the DHCP request isn't just relayed from the LAN port to the WAN port. Instead, the LB2120 seems to put its own mac address in the DHCP request before it sends it out on the WAN side. (keep in mind that the management interface on the LB2120 already has a statically assigned IP). When the LB2120 receives the DHCP response on the WAN side, it relays the IP it gets to the device on the LAN side. However, it doesn't just relay the DHCP reply it got from the WAN port. It seems to build a new DHCP reply to send out to the LAN port device. The DHCP info it sends to the LAN device has the netmask hardcoded to 255.255.255.0 regardless of the netmask in the original WAN side DHCP reply. Also, it seems that the DNS server IP is hardcoded to xxx.xxx.xxx.2 (eg 192.168.1.2 if the DHCP assigned WAN IP is 192.168.1.xx). So even in bridge mode, it looks like the LB2120 makes itself the DNS for the device connected on the LAN port.

So as best as I can tell, this is how bridge mode works on the LB2120. The path between the WAN port and the LAN port isn't strictly routed or firewalled, but the LB2120 replaces the layer 2 part of all packets as they move between the 2 bridged ports.

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LB2120 - Bridge Mode Issues