Assuming CGNAT, sure, it is the ISPs problem. But that just underscores the utility of IPv6 where defective CGNAT results in a hair pulling experience for the

Assuming CGNAT, sure, it is the ISPs problem. But that just underscores the utility of IPv6 where defective CGNAT results in a hair pulling experience for the customer. Things don't get forwarded, and there isn't a thing you can do about it except call the ISP (provided you were able to troubleshoot it to that point in the first place). If NAT is done on the router, then it gets to make a decision on every packet regardless.