Using own OpenWRT router with Vodafone/Cityfibre

For something very different this time, a quick recipe for a very specific situation.

One of the very few fiber-to-the-home solutions in the UK is through Cityfibre, sold as Vodafone “Gigafast” packages where I live. As with other ISPs they give you their own internet router. If you would like to use your own, here’s how.

TLDR; Obtain PPPoE credentials specific to your account from Vodafone, then use them to configure your router, you need to tag all your packets with VLAN 911.

1. Obtain PPPoE credentials from Vodafone. You can do so by using the chat feature on their website. Here’s an example chat transcript pasted here for your enjoyment and pleasure
 Tobi : I’m TOBi - Vodafone’s chatbot Ask me anything, I’m here to help.
You : hi, I would like to swap the provider router for the own router, could you share the PPPoE credentials I would need to use ? thank you
Tobi : Please try asking your question again using fewer words, making sure you only ask one question at a time. You’ll be able to ask other questions later if you need to.
You : ppoe credentials
Tobi : TOBi is thinking...
TOBi : Sorry, I didn't understand the question. Can you please try rephrasing?
You : own router ppoe credentials
TOBi : Sorry, I didn't understand the question. Can you please try rephrasing?
You : can I speak to a human please ?
You : xxxxxxxxxxxxxxx
You : xxxxxxxxxxx
TOBi : Please tell me what type of account you have with us:
TOBi :
Pay Monthly
Pay As You Go
Pragya : Hi , you're now connected to Broadband chat support. Please give me 1-2 minutes as I quickly go through your conversation so that you don't have to repeat yourself.
You : Hi, perfect, thank you :)
Pragya : Cyryl , Thank you for waiting
Pragya : As per the pre chat , I understand that you wish to have PPOE details , right , Cyryl ?
You : that's correct, I'm trying to establish connection using own hardware
Pragya : Cyryl , Thank you lets do a quick security check ,
You : account number xxxxxxxxxx
Pragya : Thank you very much
Pragya : I have your account ready. Let's complete a quick security check so that I can try to resolve this for you.
Pragya : Confirm the 2nd and 3rd digit of the security PIn ?
You : xxxxxxx
Pragya : Thank you let me verify it
Pragya : Brilliant that matches the record
Pragya : Would you mind being online for few minutes, while I am checking the details ?
You : no problem, thank you
Pragya : Thank you , Moving away from the chat window or not responding for 8 mins may result in losing the chat. If it does, here's a link to get back to us faster than usual: https://www.vodafone.co.uk/bbservcd
Pragya : Thank you for waiting
Pragya : Password is - XXXXXXXX

1. armed with that knowledge you can configure your router, I’m gonna give an example of configuring an OpenWRT router via their GUI interface LuCI
• -> Network->Interfaces
• note down the physical interface of current WAN interface - (WAN->Edit->Physical Settings), e.g. eth2
• remove existing WAN interface, press save and apply
• add a new PPPoE interface:
• Name: WAN
• Protocol: PPPoE
• interface - type in the custom field - name of your physical interface followed by a dot and 911 indicating the VLAN number to tag packets with - e.g. eth2.911
• -> create interface
• Create / Assign firewall-zone: WAN