We sell internet BW to small businesses, Because of how caching & peering services are configured our customers can only benefit from our local GGC and akamai node, with speed and performance only if it they use our DNS service. But despite multiple requests our clients use global google DNS services (22.214.171.124) or their own internal DNS servers and then complain about not getting decent speed or high latency. We keep telling them but it's not making any difference. One of the things we did to some of our clients who are connected through Mikrotik CCR routers is NAT their DNS requesting. Doesn't matter whatever DNS server they are using, we NAT the DNS requests to use our own DNS Servers. So far, in the last month or so, we haven't had any problems. We want to do the same thing Cisco switches/routers, Any idea how to do this?
Perhaps something like this?
Also, as someone has mentioned, if I was your customer, I would leave. Altering customers' traffic without their consent is big no-no. Beside, you're selling Internet BW. If they're using other DNS, that would mean they generate more traffic and they have to pay more isn't it?
Anyway, good luck.