I bought a Unifi Dream Machine (UDM) and then I called my ISP to change the modem to bridge mode, but they don't provide support for that. You should do it by yourself.

The simplest possible explanation of what we are doing in this post is: we're going to redirect the fiber signal from the ISP's modem to our UDM using an ethernet port of our router and the PPPoE is going to be resolved in our UDM.

Here I'm gonna show you a brief guide using my modem RTF8115VW provided by Movistar. I'll assume you know the basics and how to go to the control panel of your ISP's modem.

Step #0, the backup

I couldn't find the backup configuration page on my modem, but if you have luck you have remembered to make a backup. It's nice to have a lifeguard if you make a mistake.

Step #1- the WAN interface

At first we need to remove the PPPoE WAN interface because the WAN connection is going to be resolved by our UDM.

💡 Remember to copy both user and password of your PPPoE connection.

Just select the PPPoE connection type in the grid and click on Delete. This operation can take up to a minute.

🧠 Take note of the VLAN of your PPPoE connection, you'll need it later.

Step #2 - Ingress Filtering

Now we're going to associate one ethernet port of our modem to the WAN port using the proper VLAN. For this, I used the ethernet port #4 for this, but it could be done with any eth port. Just check the eth0.4 and click on delete to remove the current LAN configuration for this port.

Then, click the Add button to associate the port to the WAN interface with the following values:

Order: Lowest
Ingress Interface: your selected Ethernet port, eth0.4 in this example.
Associated Bridge: Here just select the WAN interface.
Ingress Packet: All the packages. 😬
VLAN ID: the VLAN number that you copy from the previous step, 6 in this example.

Then hit Apply, to complete this step. If you want to know more about ingress filtering you check it here.

Step #3 - Egress Marking

We're going to mark the packages from the ethernet port number 4, but first we should delete the current configuration for this. Just click the eth0.4 interface and then on the Delete button.

Now, we're going to mark the packages from eth0.4 with the following values:

Ingress Interface: your Ethernet interface, eth0.4 in this example.
Associated Bridge: select your WAN interface.
Accepted Type: Tagged.
VLAN ID Re-Mark: -1.
Priority Re-Mark: 1.

💡 But is not -1 a misconfiguration value for VLAN ID? 🤔 Well, I understand that this misconfiguration value overrides the 6 value that we used in our end. If you know more about this please comment, I want to hear from you! 💬

If you want to learn more about egress marking you can check the QoS Packet Marking page from Cisco.

Step #4 - the Wi-Fi

The most simple step with our modem, just turn off the Wi-Fi radio for 2.4 and 5 GHz and reboot your

Step #5 - Ubiquiti

Now, we need to configure the WAN interface in the UDM, for me this is here: /network/default/settings/internet. Click on the WAN interface and let's configure it.

Advanced: I used Manual configuration.
VLAN ID: you should enter the same number used in the modem configuration, 6 in this example.
IPv4 Connection: PPPoE in the scenario.
Username and Password: credentials for your ISP.
DNS Server: you can use Auto, or Cloudflare (1.1.1.1), or Google (8.8.8.8), or anyone you want.
IPv6 Connection: I used Disabled for me.

Finally submit your changes and wait for the internet connection to be established in our Unifi network. 🤞

Conclusion

We've learned a few networking concepts that could be used to configure several modems in bridge mode, but be careful because different models or providers could require a little tweaks to make it work! And if you have another router, like a TP-Link, the PPPoE configuration is similar to what I show you in the UDM panel.

I want to thank Salvathore, because I couldn't make it without his video tutorial.

I hope you enjoy your UDM as much as I do. 🖖