I have 2 independent network providers to provide redundancy and load balancing. I am using a LinkSys RV042 at the moment, with mixed results. It does not load balance worth a dime. But dose an adequate job of fallback.
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If SME is able to do this I am planning on a new installation.
SME does not currently have such a feature. It's something I've done some preliminary research on, and may one day get around to implementing. If anybody wishes to sponsor me to develop the feature they should contact me via email.