I work in an office complex that has two seperate ADSL connections, which they use to provide two seperate networks (actually both the ADSL routers go into a Cisco managed switch with two VLANs, one for each ADSL connection).

Circumstances have changed so that 95% of the users are all on one ADSL connection. It would be great if there were a way to join together both connections to emulate a single connection at double the speed, but the ISP doesn't support bonding.

So, is there a sensible way to take two completely seperate ADSL lines and use them to provide a single internet gateway?

5 Answers 5


There are numerous appliances and routers (e.g. Zyxel P-663H) which will support two internet connections and balance outbound sessions across the two links.

For IP routing reasons this almost always involves NAT, although if you're already using NAT that shouldn't be an issue for you.

Balancing inbound traffic resiliently is a lot harder, since you have to publish an IP address for each inbound service, and those IPs can only ever be associated with one link.

The alternative is to swap to an ISP who can bond lines so that each line is properly bonded.

  • We are (unfortunately) a bit limited in our choice of ISP as we are in a rural location that is not well supported. There are also other factors that would make it difficult to change. Hence, I want to try to do what I can with the two connection we already have.
    – Tim Long
    May 13, 2010 at 13:12
  • so what's the nature of the traffic? web access and e-mail only, or other inbound services too?
    – Alnitak
    May 13, 2010 at 13:31
  • and FWIW, if you're on IPstream (which you almost certainly are) then ISP choice shouldn't be any problem - almost every UK ISP supports that wholesale product. What you won't have are the "LLU" providers - Talktalk, Sky, etc.
    – Alnitak
    May 13, 2010 at 13:34
  • <sigh> THe office manager decided to commit themselves to another 12 months of BT misery, so my hands are somewhat tied. The solution of an aggregating router seems like the way we will have to go, at least for the short term - so I'm accepting this as my preferred answer. BTW, nice horsehead image, Alnitak. I have one that I took myself with my telescope :)
    – Tim Long
    Aug 3, 2010 at 15:40

You can use loadbalancing. When new connection appears, loadbalancer decides which ADSL line will use. Basic algorithm for choosing line switches between them for each connection. For example first connection goes thru line A, second thru line B, third thru line A, etc.

This algorithm needs support only on your site - your router. For example Mikrotik or any linux based machine.


With linux is pretty easy ( http://lartc.org/howto/lartc.rpdb.multiple-links.html )

If you want you could use pfsense for a graphical interface gateway.

Or debian + shorewall ;)



Ecessa makes some reasomnably priced load balacer/failover devices. Worth a look



It would definitely be worth considering a fully bonded connection. It would obviously mean changing public IP etc, but your new IP would be spread across the two connections, not dependant on either line seperately.

And if you went for a bonded provider that does it without using MLPPP then you don't have to worry about linux boxes or unsupported equipment in the future.

It will work on any BT line that can have broadband. Are there any more specific reasons why you wouldnt want to move ISP?

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