Howto: Multirouting with Linux

Through the magic of routing, the Internet can supposedly withstand intermittent outages, congestion, and other nasties. This is because, in theory, any network has multiple paths to any other network,” sort of like highways between cities. By having multiple connections to the Internet from different providers, a network can distribute the load over all of them, choosing the best available route for each packet. This is called Multihoming, and the protocol that makes possible is BGP. The problem is that the network needs an AS number for BGP to work, and no consumer-targetted broadband connection supplies one.

Typical small network configuration

Most networks that use regular broadband as their Internet connection have the following configuration. The service provider provides a DSL or Cable modem, and hooks it up to one computer to be designated the router. They generally give one or two IP addresses which may be static but are more likely dynamically allocated with DHCP. This router, which is often a computer but could also be a special-purpose appliance from Linksys or similar, sits between the network and the Internet connection. It usually runs a DHCP server on the internal network, assigning private IP addresses to all of the computers on the network. It also usually does NAT so that the computers on the Internal network can access the Internet. This works pretty well, but it's all a bit of a hack in order to get around the problem of IPv4 address exhaustion. With NAT, the entire internal network hides behind the router, so while a remote server thinks it's dealing with the router, the router knows to pass communication along to the correct internal computer.

The solution hack

There is a way to get similar functionality by using multiple broadband connections without an AS number or any other cooperation from the service providers. It's generally referred to as "Multirouting."

 Source : http://lukecyca.com/2004/howto-multirouting-with-linux.html