It's harder for a transit ISP, but your average, typical ISP isn't providing transit for a huge number of other AS's. We ($work) are pretty small but we provid

It's harder for a transit ISP, but your average, typical ISP isn't providing transit for a huge number of other AS's. We ($work) are pretty small but we provide transit for a handful of other AS's and I have extensive filtering everywhere. Downstream customers can advertise networks that are allocated/assigned to them and I'll accept IP traffic from those source addresses. If they send us traffic with random source IP addresses, though, it gets dropped by the router of ours that they are connected to.