Hello Hisham
It is indeed true that DNA is a difficult topic to study for, when approaching the CCIE because of the fact that, as of today at least, it’s not available in any simulation. Here are a few of Rene’s answers that may help you out:
https://forum.networklessons.com/t/hi-im-henry-from-hong-kong/10278/3?u=lagapides
I hope this has been helpful!
Laz