[sllug-members]: Multiple DSL Lines

[Corey Edwards]

On Tue, 2006-12-12 at 17:27 -0700, Mac Newbold wrote:
> > That should work just fine as long as the DSL lines are from the same
> > ISP. With upload traffic, your router gets to decide which interface to
> > send packets out on so it can do whatever kind of load balancing you
> > want. The reason to use the same ISP is that many (hopefully most, but
> > unfortunately not all) will refused to send out packets from an IP
> > address that shouldn't be on their network. They do that to prevent DOS
> > attacks mainly, but it's just good practice in general.
> 
> One thing to note is that since they're not "bonded", they'll have 
> different IP addresses on the other end, and so traffic on a TCP 
> connection for example will most likely come back to the same IP it came 
> from. If you're running a server behind it, your DNS will point at only 
> one of the two incoming IPs generally, so all the incoming will hit the 
> same line. Each connection, once started, will be tied to a particular 
> line, and no connection will be able to use both lines. So if you're 
> trying to do a single really big upload or download, you'll still see the 
> same performance you do now on that transfer. But any other transfers will 
> see much better performance unless they're already tied to that same link. 
> Anything starting up after the one link is tied up will generally use the 
> other (unless it is an incoming connection, in which case the balancer 
> won't have a say in it).

You're correct. Without bonding the lines, the incoming traffic won't be
load balanced. Adam's original question was about upload so I hadn't
mentioned it. He can send out from whatever IP he wants, over either
line. He just needs a router that's smart enough to do that. I've never
gotten it to work right with Linux, but that could just be me.

To set it up the other direction is pretty easy on a Cisco router at
least. It would require having an additional block of IP addresses which
would be the actual addresses that get load balanced. The IP on each DSL
modem would not be. This is the part that would really require a good
ISP.

In our case we're using Cisco routers on both ends, which makes it quite
simple. The hard part is getting that many good quality copper lines
into the building. For whatever reason some lines come in at 7,500 feet
and others are at 11,000 feet.

Corey