I've just bought an Asus RT-N66U wifi router which connects via the WAN port to a TP-LINK TD-8817 ADSL router and on to the internet.
The Asus is set up to do DHCP etc and can be considered the 'main' router in my network. All access to the internet must go through the Asus.
My question: both the Asus and the TP-LINK have built-in firewalls. Should I enable both? Will there be a significant performance hit if I leave both enabled? If I should use only one firewall – which device?