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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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up vote 6 down vote accepted

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. – Matt Dec 12 '10 at 21:10

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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