BT business I believe. What I am trying to figure out at the moment is if I download traffic destined for my singular Public LAN IP and the ISP receives the traffic initially on their LNS (still destined for my 1 public LAN IP), the ISP then load balances across the 3 lines to get to my end so why would that not allow the speeds to be added together?

Load blaancing works in a number of different ways. If it is a connection that has been opened up then that connection will probably be set to always use the same path - if this isn't done then it can cause all sorts of problems trying to put it together again at the other end.

Most connections on simple load balancing use round robin. The first connection goes through pipe A. The second pipe B. Third pipe C and 4th back to pipe A. And it would keep doing this with no reference to how busy the pipe is and once a connection is open it would always go down the same pipe (so a connection to do a 10GB download would always use the same pipe).

An extension of this is to select the pipe used by the one that has most available capacity. But, a single connection would still stick with the same pipe.

Full load balancing would be packet level using the least used pipe for each individual packet. This requires a lot more intelligence and processor power. It also requires both ends to be powerful to be able to ensure packets are reassembled in the correct order.

Load balancing is a complex area and many systems (especially relatively low cost) will go straight round robin. There is the potential with round robin that the first connection is a 10GB download, second is 1KB, third 1KB and 4th back to 10GB. That means 20GB would come down 1 pipe and 1KB on each of the others - this can happen unless you invest in more expensive capabilities on load balancers.

The lines are load balancing equally and all 3 lines are the same 40mb/s down and 10mb/s up so it should be working fine. I am probably missing something here but as far as I can tell I should be getting 40 x 3 mb/s download as the ISP's LNS is receiving traffic destined for my 1 public LAN IP and then load balancing across the links.

Have you discussed your setup with BT support, i.e. do they confirm this should work?

There is a reason why the providers who support bonding in its various forms shout about it, i.e. it is fairly rare and I did not think BT Business Broadband of the GEA-FTTC or ADSL2+ variety supported bonding.

Load balancing is a different matter as you do not need ISP side support for that.

They have said that it should work, they also have had to configure a device on their side (the LNS) to load balance traffic they receive from the internet across my lines so as far as I can tell it should work.

I'm getting the impression that you have no idea what you're doing here .

I've never heard of BT retail offering bonded solutions or MLPPP support.

Your best option to test this is to do VPN bonding with a datacentre somewhere. This can be done regardless of ISP/router config.

Get a PC with two/three ethernet ports and a cheap gigabit VPS in the uk and tryy it out: http://simonmott.co.uk/vpn-bonding

How are you testing with only 1 PC, what spec Ethernet are you using to get to the router?

If the PC is only using 100Mb the router won't work much faster than 50Mb. (Make sure the PC is using 1Ge over a Ge port. (Not 100Mb over a Ge port)

Likewise the router is being set to use the Ge port as Ge not as 100Mb.

Are you downloading multiple things at a time the othe end may be restricting it if only one, ( Many sites will not stream at 100Mb to one end point).

(I am assuming the PC has the umph to actually do more than 100Mb itself with at least 3 download threads)

If the PC is only using 100Mb the router won't work much faster than 50Mb

Why? The PC on an ethernet 100Mb should be full duplex and therefore can very nearly max 100Mb. If the PC is only running half duplex then that in itself is a problem that needs to be fixed.

You rarely fill to the full capacity so a 100Mb ethernet port usually will max out at around 80Mb.

Which a tad more than "not much" over 50Mbps .