I have to admit some misconception and misunderstanding. Yes, the term Load
balancing describes a functionality in a router that distributes packets
across multiple links based on layer 3 routing information. To achieve fault
tolerance two approaches could be used: hot standby and synchronized
execution.
So I've re-read a couple of Shindler's books and dug through
www.isaserver.org and found that Tomas Shindler stated:
"As things stand at this moment, you can't use multiple external interfaces
on the ISA Server. At the time the book was written, Microsoft was implying
that you could install multiple external interfaces on the ISA Server and
allow ISA Server to use them in a fault tolerance situation. Since that
time, its become clear that multiple external interfaces isn't really a
viable solution.
One option is to use a hardware load balancing in front of the ISA Server.
There are router solutions that allow you to plug multiple external
interfaces into them to create a fault tolerant solution. You can also use a
hardware load balancer, such as F5 networks BigIP. These solutions work
well, but suffer from being somewhat expensive.
However, if you can wait a month or two, something really exciting is coming
around the corner. Rainfiity (
www.rainfinity.com) is coming out with a
product called RainConnect. The RainConnect product will first be
implemented as a second server that you can put in front of the ISA Server,
and later will be implemented to integrate with the ISA Server machine
itself. This will allow you to connect multiple external interfaces into the
computer, such as a DSL and T1, or multiple DSL or T1 lines. The RainConnect
product will automatically load balance the connections and provide fault
tolerance. It's a very exciting product and I expect it to be quite
popular!"
Evaluation versions are available at
http://www.rainfinity.com/products/downloads.html.
Through IntelliNAT and AdaptiveDNS, RainConnect is able to fail-over and
load balance ISP connections without reconfiguring routers or obtaining
special cooperation from your ISP as is required with BGP-based solutions.
At the same time, FAQ section explains that "None of the solutions that do
not use BGP provides transparent fail-over of connections. When RainConnect
fails-over an ISP link, existing connections are broken. RainConnect will
ensure available ISP links are used for new and re-established sessions."
Probably the best solution is choosing the Cisco products as always.