With Asterisk on the router, I have noticed I was getting a ton of brute force registry requesta. Since then I have set my iptables to only accept incoming connections from my SIP providers and drop the rest.
I used to drop from a cron job checking the log but this didn't prevent the router from grinding to a halt for the five minute interval between when the job ran.
My question is if some is hitting the router with a bruteforce attack and the iptables drop the connection, what sort of CPU slow down would be caused to the router? When it hit my asterisk server it basically killed the router in terms of responsiveness.