Using multiple upstream providers
It is possible to balance the network load among different upstream providers, but cooperation is needed from the host OS and networking infrastructure. You need to use different tcp_outgoing_addresses to mark the packets intended for different upstream providers. You then need to use policy routing on the host running Squid to correctly route packets to the correct uplink provider. Some information on how to do it on Linux systems can be found at http://lukecyca.com/2004/09/28/howto-multirouting-with-linux/.
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