How to Bridge ISP-provided Gateway to Router
By: Eugenio Pastoral
Tags: Bridge Mode, Pldt, Double Nat, Networking, Router

UPnP fails because of Double NAT

I’ve been dealing with Double NAT over the past few months. I’m well aware that my initial configuration will cause this issue since both the gateway and the router are acting as routers. Having it this way would cause problems for certain services that require an open port, or if you play games like me, your NAT type will show up as Moderate or Strict.

What stopped me from fixing this for such a long time was because of the admin privileges of my gateway. My ISP locked down access to configurations aside from Wi-Fi settings, so I couldn’t set the gateway to bridge mode. Customer service didn’t help either. Although, they finally gave me access after six months. But how did I fix this? Let’s talk about what NAT is first.

What is Network Address Translation?

Essentially, network address translation (NAT) allows a single device, like a router, to act as an agent between the public network and a local network. This allows devices in the local network to share a single IP address with anything outside the private network.

What’s happening here, to put it simply, is that the router translates your devices’ local IP addresses to a single public IP address so that you can communicate on the Internet. In the case of having Double NAT, it will perform this translation twice, which may cause issues with UPnP and other services that may require open ports for remote access or whatnot.

It’s also possible that you may have CGNAT, which may be beyond what this guide addresses.

Overview

I’ve tested this with the gateway, Prolink PRS1841U-v2, provided by PLDT, and a third-party router, ASUS RT-AC59CU V2. It’s worth noting now that I’m using VDSL2, and the guide is tailored for such. There may be slight differences in the settings you should modify depending on your ISP, but for the most part, it should be similar.

The Guide

Just a quick disclaimer before we begin. Do note that I’m not responsible if things go wrong. Proceed at your own risk.

Part 1: Setting up the Connections

Cabling for Gateway to Router

Ensure that your gateway is connected to your ISP via RJ11 or fiber optic cable. Connect your LAN port (maybe LAN1 depending on your port bindings) of your gateway to WAN port of the router.

Part 2: Setting the Gateway to Bridge Mode

To start things off, we have to set the gateway to bridge mode. Doing this will make your gateway solely act as a modem.

Prolink PRS1841U-v2 PTM WAN Settings for Bridge Mode

Login to your gateway. In my case, it’s 192.168.1.1. Go to your WAN settings. Now, you may find two options: PTM and ATM. Choose PTM. My gateway, by default, is using IPoE as the channel mode to connect to my ISP’s servers. Toggle Enable Bridge. Under bridge mode, select Bridged Ethernet (Transparent Binding). Disable NAPT by toggling it. If you can’t find this, look for NAT and disable it. Go to your DHCP settings and disable the DHCP server. Go to your WLAN settings and disable wireless LAN interfaces. Commit and reboot.

Part 3: Configuring the Router

At this point, you might have lost your internet connection. This is because your gateway is now functioning only as a modem. What we have to do next is to set up the router so that it can do the routing.

ASUS RT-AC59CU V2 WAN Settings for Transparent Bridging

Login to your router. In my case, it’s 172.16.0.1. Go to your WAN settings. Under WAN Connection Type, set it to Automatic IP. Toggle Enable WAN. Toggle Enable NAT. Toggle Enable UPnP. Depending on your ISP, you may have to clone the MAC address of your gateway to be assigned a WAN IP address. ASUSWRT conveniently has this feature. Apply the settings.

Other Notes

At this point, you should have internet connection again, and Double NAT should be gone. If it’s not, contact your ISP.

Regarding PTM and ATM, select ATM instead if you’re still using an ADSL gateway. Open your ATM profile and set the channel mode to 1483 Bridged. After which, just continue from Step 7 in Part 2.

To reiterate, some of the settings you need to change might be different depending on your ISP. For example, your gateway might be using PPPoE instead of IPoE. You may also have a static IP as opposed to the dynamic IP in this guide. Make necessary adjustments for your own network topology.

References