The pain and grief imposed on the network administrator by having to tarnish his sparkling network with an abominable ISP-imposed DSL router can sometimes be somewhat ameliorated by asking the ISP to deliver a bridged circuit instead of a routed one. (Read here about the difference between routed and bridged DSL circuits.) There are two different possibilities, each with its own problems:

Bridging with PPPoE

Sometimes the service model on the line may consist of the routed IP link going through PPPoE, which runs over an auxiliary, usually virtual Ethernet which is bridged over the DSL link. This auxiliary bridged Ethernet carrying PPPoE is completely virtual when you use a one-box-does-it-all router, but it can be made real if you can find a dumber CPE that does non-IP-aware Ethernet bridging over DSL and nothing else. You can then put your own router behind that dumb bridge, and that router would be completely yours, its WAN interface being PPPoE.

The problem with this solution (besides the difficulty of finding suitable dumb bridge CPE, see below) is that the ISPs are moving away from PPPoE. When we tried this with MCI/WorldCom's DSL Office service in late 2004, they had said that they didn't do PPPoE. (And this was in the good old days before they became Verizon and when they still operated Copper Mountain DSLAMs of the old Rhythms network.)

Why are ISPs moving away from PPPoE? Well, one objective problem I can see with PPPoE is that it results in the WAN IP link running with an MTU of less than the Internet de facto standard of 1500 octets. While I may be willing to live with that, others may not be, so that could be one plausible reason why the ISPs are moving away from PPPoE. But in the end all that really matters is that they are.

Bridging without PPPoE

In bridging without PPPoE there is no routed IP point-to-point link at all. Instead the ISP gives you an Ethernet with a netblock assigned to it, bridges it over the DSL link, and makes their backhaul router appear on your Ethernet at one of the IP addresses from your netblock.

The problem of course is that an Ethernet that's bridged over a DSL link and goes to some sight-unseen router in another dimension is no longer yours. Instead it is more of a throwaway Ethernet, and the netblock assigned to it is therefore also a throwaway netblock. Thus if you have a Class C NET, and then you get a bridged SDSL circuit and assign that NET to it, you have just wasted your Class C NET. Not good.

Bridging with a throwaway netblock

What we ended up doing on the MCI/WorldCom SDSL circuit we had in 2005 was a two-Ethernet solution with a good Ethernet and a bridged throwaway Ethernet. The throwaway Ethernet was bridged across SDSL, and our good Ethernet was separated from it by an Ethernet-to-Ethernet IP router (chosen, configured and operated by us). Our prized Class C NET was assigned to the good Ethernet, but the bridged throwaway Ethernet required getting another throwaway netblock from the ISP (it was a /29 in our case). The ISP's backhaul router had to be configured to assign the throwaway netblock to the throwaway Ethernet, but also to know about our other netblock (the Class C NET) and to IP-forward it to one particular IP address on the throwaway Ethernet, the one assigned to our E-to-E router. The last part was very unusual for the ISP and took a lot of yelling and screaming on the phone to get them to do right.

The obvious problems with this solution are that it's very unnatural, it wastes IP addresses, and it may be hard to get the ISP to agree to it because:

  1. The ISP may rightfully object to the waste of IP address space,
  2. This arrangement is so out of the ordinary that it may be very hard to get the ISP to understand what you want, let alone to get it right.

Finding bridge CPE

One final problem with the bridging workaround is finding a CPE device that acts as a non-IP-aware Ethernet bridge. Most DSL modems nowadays are actually routers, non-IP-aware bridges are going out of fashion along with PPPoE. The solution we had with MCI/WorldCom SDSL in 2005 was on a Copper Mountain SDSL line and used a CopperRocket bridge, one of the very few pieces of DSL CPE that are somewhat tolerable. With Verizon having dismantled the Rhythms DSL network, this can never happen again.

With true dumb non-IP-aware bridges going out of existence, when an ISP like Verizon Business does offer a bridged circuit, the bridge CPE they now use is actually a router dumbed down to operate in a bridging mode. But that is not the same thing — a feature-laden router dumbed down to a bridge is still a feature-laden router and will still be a thorn in the network administrator's side.