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I have a DrayTek 130 VDSL2 modem, which uses PPPoA for connection. My ISP requires CHAP for authentication, but I am using an alternative OS on an external router, whose PPP plugin does not work as one would expect, thus having trouble with establishing a PPP link on my ISP's network.

Before I could paraphrase my question, I'd like to make it clear what I am trying to achieve:

  1. Eliminating the need for PPP authentication on the external router by saving the login details on the modem.
  2. Disabling the LAN and delegating the public IP to the external router.

As for the first objective, I have found the corresponding options in the console, allowing me to save the login details, but unfortunately it turns the device into a router and I can not use the public IP on the external router for NAT.

As for the second objective, I have turned on the MPoA with bridged LLC, allowing me to delegate the public IP to the external router, but in this case I have to use PPP and CHAP on the external router for authentication.

Unfortunately I am unable to do both in the same time.

First of all, is it possible at all, to delegate the public IP to an external router without having two public IP addresses?

This modem offers three MPoA bridging protocols: LLC, VC-MUX, IPoE. My question is which one could solve both my problems?

Tony
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1 Answers1

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This modem offers three MPoA bridging protocols: LLC, VC-MUX, IPoE. My question is which one could solve both my problems?

I would say none of them. They're all about bridging the raw VDSL line, and that's mutually exclusive with having the modem do PPP.

PPP authentication doesn't merely unlock the connection (unlike say 802.1X) – instead it establishes a tunnel (almost exactly like VPNs) which has its own IP configuration, and all IP packets must be encapsulated inside that PPP tunnel rather than being sent directly.

The problem is that a PPP tunnel only carries bare IP packets, without the Ethernet frame header, thus it is not bridgeable with actual Ethernet (or anything else). In addition, the IP address is assigned as part of the PPP handshake – there's no DHCP service for your router to use.

is it possible at all, to delegate the public IP to an external router without having two public IP addresses?

In theory yes. (Though you already ruled out the usual way of making the other router directly act as the PPP client – this would be easiest.)

Sufficiently flexible devices could set up an unnumbered PPP tunnel and allow its IP address to be assigned to another router (probably in combination with Proxy-ARP), but I'm not sure if this is an option on your current modem. I've seen LTE modems do it.

u1686_grawity
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