Wifi router + ADSL modem: Where to run the firewall?

Question

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?

Answer 1

There shouldn't be a performance hit if you enable both, but if you only want to enable one I'd recommend the one on the ADSL router.

Why? Because this means that anyone trying to get into your network is blocked at the point of entry. If they can get onto the ADSL router then they could try to attack your Wifi router - which could in turn compromise your entire network.