If you have links from both ISP A and ISP B and decide to send traffic out ISP A's link sourced from addresses ISP B allocated to you, ISP A *should* drop that traffic on the floor.
This is a legitimate and interesting use case that is broken by BCP38.
I don't agree that this is legitimate.
Also we're talking about typical mom & pop home users here.
There are SOHO modems that will fall back to a second connection if the primary one fails, but that's not what we're talking about here. The customers I'm talking about are businesses large enough to have two dedicated upstreams, and a chunk of address spaced SWIP'ed from each. Some run BGP but I get the impression as likely as not they have static routes to the two upstreams. For people who missed it the last time, I said $50K/mo, not $50/mo. Letters matter. R's, John