Using an ADSL modem/router as modem only

Question

I have a D-Link DSL-2520U ADSL modem router and a Netgear WNDR3700v2 router. I want to use the DSL-2520U as ADSL modem only, and the WNDR3700 for NAT, firewall, DHCP and wireless access.

How I should setup the DSL-2520U to act as modem only? In particular, I don't understand the ADSL configuration required.

Details

The DSL-2520U is an ADSL modem router, with a single ethernet port and no wireless capability. It does have NAT, firewall, DHCP, and more things that I dont' need. For reference, I've put its help pages here and here.

I have OpenWRT installed on the Netgear WNDR3700 router. In my current setup, the DSL-2520U does NAT and firewall. The Netgear router has its firewall and NAT disabled (to avoid double-NAT). It does wireless and DHCP.

I want to change this setup such that the DSL-2520U only acts as the ADSL modem. I want the Netgear router to take over the NAT and firewall responsibilities, so that it can then be setup with pretty-much the default OpenWRT configuration. In addition, I still want to be able to access the DSL-2520U admin page.

Edit:

I'm happy to go without access to the DSL-2520U admin page if that allows for a better setup.

Solution

Whatever you do to get your Netgear router to handle the connection directly will prevent you from accessing its configuration page. There is absolutely no way to overcome that limitation which you impose on yourself.

Your best solution to keep access to the modem-router's configuration page would be to enable DMZ from the modem-router pointing to your Netgear router. This would give you all the routing and firewall flexibility you could need on the Netgear router.

Update based on question update: If you do not need access to the modem-router's configuration page, the best solution to give full control to your Netgear router is to put the modem-router in bridge mode (which is present and documented in its manual).

Refer to your modem-router's manual for reference.

Comments

[Run CMD]

Not all devices support modem / bridge functionality. Some do, but not officially. snbforums.com might be a good place for device specific Q+A.

[cmangla]

A text version of my current ADSL configuration is available here

[Run CMD]

Nothing prevents the OP from introducing a switch between the DSL modem and the router and accessing the DSL modem's admin page from within the LAN segment between the DSL modem and the router, no?

[Julie Pelletier]

Actually no because OP wants the Netgear to do a direct connection which is PPPoE, which would prevent the router from controlling its access to the WAN. It could not route local traffic on the WAN side.

[Run CMD]

If the OP has a PPPoE based DSL then the single LAN port of the DSL modem will prevent him from accessing the DSL modem's config page. But not all DSL lines use PPPoE for authentication.

[Julie Pelletier]

I agree there may be exceptions but they're quite uncommon.

[cmangla]

The DSL-2520U is currently configured to use PPPoE. I've put up a text version of the current configuration here.

[cmangla]

I'm happy to go without access to the DSL-2520U admin page if that allows for a better setup.

[Run CMD]

@cmangla If you configured your DSL device to do the PPPoE, you're not operating it as a modem / bridge. In this case, there's no problem accessing the config pages. However, setting up DynDNS, VPN and visible services in the router will not be straight forward. You'd be much better off in many cases if you operate the DSL device as a modem / bridge and configure your PPPoE WAN in your router. But as I said, not all devices support DSL modem / bridge mode.

[cmangla]

@JuliePelletier Am I correct in understanding that if I put the DSL-2520U in bridge-mode, then the Netgear router will need to "dial in" to the ISP using PPPoE?

[cmangla]

@ClassStacker Looking at the manual, it seems the WNDR3700v2 does indeed support PPPoE. The complication now is that I've replaced the firmware with OpenWrt, so I need to figure out if that has an easy UI for PPPoE.

[Julie Pelletier]

@cmangla: The Netgear router does need to handle the PPPoE connection if you want to give it full control.

[cmangla]

@JuliePelletier So once the Netgear router connects to the internet via PPPoE, will its WAN port get a public IP address?

[Julie Pelletier]

Yes. That's how it works.

[cmangla]

@JuliePelletier Ah, that's what I wanted, thanks!