Hello, what should I set TTL and MTU to for 6in4 tunnel on my ASUS RT-AC66U router?
You normally don't need to change anything unless you experience issues.
Then, MTU is not dependent on your router selection, neither is TTL.
Which are default values then? :P
MTU is on Advanced tab of your tunnel details = 1480.
That's what I set it to. But what about TTL?
TTL is used to prevent things like route loops or for putting a limit on the "network distance" you want your traffic to follow. Most OSs have a reasonable default:
https://en.wikipedia.org/wiki/Time_to_live (https://en.wikipedia.org/wiki/Time_to_live)
If you HAVE to set something, try 64. It should be fine unless you live somewhere with a VERY convoluted connectivity to the rest of the world.
Dan