[c-nsp] Load balancing across 2 ADSL lines

[Ryan O'Connell]

On 13/10/2005 11:53, James O'Farrell wrote:

>I am looking to load balanced across 2 ADSL lines either both coming
>into the same router or using 2 separate routers, any thoughts would be
>much appreciated.
>  
>

If you want to use two seperate routers you're really ideally going to 
need three devices to do this as you need to combine both the ADSL 
streams at a single point to allow NAT to work properly. (I.e. disable 
NAT on the ADSL routers themselves) I guess you already have the two 
ADSL routers, so if you can find a Linux/BSD box or another cheap 
two-port ethernet router or firewall that supports NAT you should be OK.

If you have the cooperation of the ISP, get them to route a block to you 
on both links and set "lcp:interface-config=ip load-sharing per-packet" 
on their RADIUS server to load balance the downstream traffic. If the 
far end ISP won't do this, pretty much any solution you try to use for 
loadbalancing downstream traffic is going to be a hack and rather ugly, 
although I know some people use an unencrypted IPSec VPN between boxes 
at either end of the ADSL lines which apparently works reasonably well 
if the network devices concerned can do a VPN across multiple routes. 
(Cisco can, on 12.4, using IPSec tunnel interfaces)

For upstream traffic, you should just be able to configure two default 
routes on your NAT router/firewall/Linux box/BSD box pointing to both 
ADSL routers. Depenging on the kind of box/router involved, you'll need 
to set per-packet rather than per-destination load balancing for best 
performance.

Note that the above solutions will result in out-of-order packets, 
unelss you disable per-packet load balancing which means you'll be lucky 
if you get better than 70/30 balancing. This can be a problem for 
real-time traffic, such as VoIP. If you need to do it "properly" you'll 
need a router with 2 ADSL cards in it (e.g. 1700 series) and run 
Multilink PPP - again, this depends on your ISP supporting it.