Can/should modem and router be on same LAN?

Question

I am using an Openwrt/LEDE modem/router I am happy with (Buffalo).

However, as I am transitioning from ADSL2+ to VDSL2, the Buffalo cannot deal with the DSL signal since it's ADSL only. The ISP is giving me another modem-router (ChinaWare) that works with VDSL, but it's got crippled firmware, so it can't do everything my Buffalo's doing right now.

For this reason, I'm going to use the new modem-router (ChinaWare) in modem only mode, and my old modem-router (Buffalo) in router only mode. However, the ChinaWare admin menus have no "bridge mode" option, and the Buffalo has no WAN port. Therefore, I thought that I would simply point the Buffalo's wan (pppoe) to the ChinaWare's LAN address. This, I hope, will make the router part of the ChinaWare inactive.

Here are the questions:

Suppose my "true" router - the Buffalo - has LAN IP addr 192.168.1.254.

1) Can the ChinaWare, used as modem only, be on something like 192.168.1.253, or will this wreak havoc with the rest of the LAN (computers, printers, NAS units etc.)? The alternative of setting up a dedicated VLAN and losing a LAN port on the Buffalo is much less desirable for me.

2) How do I make sure the ChinaWare never tries to hook up its WAN side (through the DSL line) to the LAN side except by opening the pppoe session? I thought about turning off NAT, firewall and all similar router options. Is there more I haven't considered? (It would be nice to have a "bridge only mode" option, but it isn't there. Talk about crippled.) If I manage to put the ChinaWare modem on the same LAN, I could even use it as another AP/ethernet switch, as long as it refers to the appropriate gateway (Buffalo on 192.168.1.254) for all WAN traffic.

Thanks for any help. I'll be glad to study any relevant material you might offer!

Answer 1

I solved the problem, but I didn't post a satisfactory answer. Here it is, albeit a bit late.

Both router and modem are running OpenWRT, so their physical ethernet ports can be reconfigured as needed. All the necessary configuration could be performed via OpenWRT's http(s) based config interface, LuCI: no direct editing of system files was needed.

The modem was set to work in bridge mode, so no routing is happening there (no iptables package needed, etc). See for example https://forum.openwrt.org/t/wan-dsl-bridge-mode/912/13 . Furthermore, in the switch configuration page, I reconfigured its WAN port as just another LAN port.

The router was originally born as an ADSL modem-router, so it doesn't have a WAN port at all - only 4 LAN ports.

1. Can the ChinaWare, used as modem only, be on something like 192.168.1.253

Yes, it can. On the router, I created a new interface called modem that opens a PPPoE session. I used one of the router's LAN ports and assigned that port to an unused VLAN (say, VLAN ID=5).

On the modem, I created the dual side of the same interface: same VLAN 5, but I did use the modem's original WAN port (reconfigured as plain LAN just as the other 4 ports via OpenWRT/LuCI switch config page).

2. How do I make sure the ChinaWare never tries to hook up its WAN side (through the DSL line) to the LAN side except by opening the pppoe session?

Easy after I figured it out! The ChinaWare modem doesn't know anything about routing to WAN at all. All it knows is its own modem interface, which is a bridge including VLAN 5 and its modem hardware port (protocol: unmanaged, eth0.5 bridged to dsl0).

On the modem, there is another "proper" LAN interface (VLAN 1) with protocol: DHCP-client, so the modem gets its LAN address from the router at startup (that is, 192.168.1.253 in my initial question). I set up a static lease for the modem's IP address on the router.

In the modem's switch config page, I added both VLAN 1 and VLAN 5 to the same used-to-be-WAN port, both tagged. As a result, the 4 original LAN ports are available and only carry VLAN 1 (untagged), so they work as a plain switch.

The final result is just what I wanted. I have the modem's management interface (LuCI) on the LAN address 192.168.1.253. All WAN traffic to/from the LAN goes through the router (via the PPPoE opened by the router through the modem on VLAN 5). The 4 LAN ports on the modem are now working as a switch.