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A friend of mine was telling about his new Draytek 120 ADSL modem and how reliable it is. He mentioned that it supports transparent PPPoE & PPPoA bridging.

I didn't get around to asking my friend at the time. Why is this useful?

Is it useful in the case where an ISP supplies PPPoA to allow the public IP normally assigned to the router to be directly assigned to a PC running PPPoE? thereby bypassing the need for NAT?

PS: I don't want you to tell me what PPPoE is and what PPPoA is. I know what that is. Why bridge them?

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Sometimes you want the modem functionality of the device, but not its routing or firewall capabilities. For example, maybe you want to use PFSense as your router/firewall but you only have the device you mentioned. By putting it into bridge mode, it allows you to use PFSense as your PPPoE device, giving you all the flexibility and functionality of the PFSense firewall withotu having to NAT to a NAT.

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    That makes a lot of sense. He did mention that he was using a WRT54GL with DD-WRT as his router. So, that explains it.
    – hookenz
    Dec 12, 2010 at 21:10
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I believe most people call this type of a setup a "half-bridge" or ZIPB (Zero IP Bridge). It works by abusing DHCP and ARP to route the PPP IP address to the LAN interface so that another device can handle the routing and firewalling.

Google shows a lot of forum postings about these key words but I wasn't able to dig up a definitive source about ZIPB.

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You mention that you know about PPPoE; one of the subtleties is that it is actually PPPoEoA when used over on DSL.

The usual mode of operation for when a router is also the modem is that the device directly initiates a PPP session using PPPoEoA, and routes accordingly.

Other options are RFC 1384 bridge, IPoA, PPPoA (this is more efficient than PPPoEoA due to less protocol overhead) and "Full Bridge" (this gives up routing entirely and simply wraps up all broadcast Ethernet packets into ATM cells and sends them to the other side: PPP is nowhere in the picture for the router-turned-modem. This allows use of a dedicated and more powerful router and reduces load on the modem). Most DSL routers allow these.

The Draytek device allows you to have a PPPoA session from the modem onwards upstream, but PPPoE between the router and modem : it's a translating bridge between Ethernet and ATM. This is the best of both worlds of efficiency and flexibility.

No other device that I know of allows this.

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