VDSL Modem in Bridge Mode: How does it work?

// under Networking Modem Internet OpenWrt

In networking terms, a Bridge is a way in which multiple networking interfaces (physical or virtual) can be made to function as as if they were a single network interface. In general terms, this is how clients associated with a router on both wired Ethernet and wireless can communicate together. In my specific case, I have a VDSL modem which is configured in bridged mode and plugging into my Turris Omnia router to enable the Turris to connect to the Internet.

With details scarce online about exactly how this black magic works to allow my Ethernet-only router to connect to a VDSL telephone line, I went investigating and here’s what I found. The tl;dr of this is that the “black magic” isn’t too hard to understand, it just operates at layer 2 (data link layer). Likewise, a key takeaway for those with similar hardware is that yes, the Turris Omnia router can connect to both IPoE and PPPoE networks, provided you have a bridge modem present and configured correctly.

Firstly, some terminology:

• ATM - Asynchronous Transfer Mode
• PTM - Packet Transfer Mode
• IPoE - IP over Ethernet (typically seen as just DHCP as a router’s WAN options)
• PPPoE - Point-to-Point Protocol over Ethernet

So in my case, I have my router configured to use DHCP to connect to the Internet and it is connected to my modem via an Ethernet cable and the modem is set up in bridge mode. What this mean, at least in my modem’s OpenWrt configuration, is that all the physical interfaces are bridged together like so:

bridge name     bridge id               STP enabled     interfaces
br-lan          0fff.b8dfe319fa2c       no              eth0
                                                        eth1
                                                        eth2
                                                        eth3
                                                        eth4
                                                        wl0
                                                        wl1
                                                        atm_8_35
                                                        ptm0

This includes the Ethernet ports, wireless LAN and the ATM/PTM xDSL connections. So this means that I can technically plug the router into any of the ports at all and it’ll work (though in my case, I’m plugged into eth0).

So my ISP uses IPoE (called just DHCP in my OpenWrt router’s configuration) to connect, which more or less boils down to the router sending DHCP messages and getting a response from the ISP (over the modem’s bridge br-lan to the PTM interface ptm0). The way in which the modem knows how to magically forward these frames/packets is by maintaining a forwarding table of MAC addresses of devices attached to the bridge. You can see this inaction by running the command:

brctl show
brctl showmacs br-lan

where br-lan is the name of your bridge interface in the modem.

So, the process goes like so:

1. The router sends DHCP discovery message out of its WAN port, which is connected to the bridge modem. This message contains its MAC address as the source.
2. The bridge modem considers what to do with DHCP discovery, which has a broadcast destination. In this case, the bridge’s forwarding table has no specific port for a broadcast and so broadcasts to all ports on the bridge. The modem records the router’s MAC address and port in the forwarding table for later usage.
3. The DHCP discovery message crosses my ptm0 link and arrives at the ISP. The ISP’s DHCP server then sends back a DHCP offer with a destination of the router’s MAC address.
4. The bridge modem receives the DHCP offer and consults its bridge forwarding table to determine what to with the message. Upon finding the router’s MAC and port in its forwarding table, forwards the message onto eth0.
5. The router receives the DHCP offer and then continues to complete the DHCP process, with messages being forwarded across the bridge in the manner mentioned in steps 1 through 4. Further data flows the same way out of or into my home network, effectively equating to the router being plugged transparently into the ISP’s networking equipment.”
6. The router now has now established an IP address and determined its default gateway, allowing an Internet connection.

There you have it, that’s how an IPoE or DHCP connection is made across a bridged VDSL modem. Once the bridge’s forwarding table is established, then that’s more or less that.

I was unsure originally because I didn’t know which port I had to plug into on the modem - anecdotal evidence online said it had to be port 1 (eth0) - but now I know it can be any port (even Wi-Fi if you could get that happening). Likewise, I wasn’t sure about how the communication should take place because my previous setup used PPPoE; in this case, the bridge was just the same, it’s just that now DHCP packets are being passed around to set up the connection compared to PPPoE discovery and PPP session data. It’s my understanding IPoE/DHCP has a lower overhead than PPPoE because of the PPP overhead imposed by the latter so that’s one benefit, as well as not needing to maintain credentials in the modem for PPPoE.

For further reading, check out Using PPPoE and IPoE in Ethernet Broadband Networks, a white paper prepared by Juniper Networks, which more or less covers all the ins and outs of IPoE and PPPoE, and now you know how the bridge modem fits into that mix.

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