no shahin ankur is not asking about how to configure deault 
root ankur is asking that what hapeen when we implemenr 
default root to that router which is directly coected to 
the isp router.
you have just expalinf that how to implement but what 
happen after that u donot explain
so here is explanation

on the router which is coneced to isp we never implemet 
default root 

now question is why

answer is so simple it produse routing loop
 now question arises how
let we implement deafult root os the router which is 
directly conected to router now any packet comes from isp 
that router forward to r2 whaich is another router
another router r2 doesnot have any entry of that ip address 
but have an enrtry about deafult root then it will agin 
send it to r1 now r1 have default root confrigation so he 
again sent it toi the r2 and process contute

so one thing always remebr the router which is directly 
conected to isp we never implement default

thenquestion is what routing protocol should we implemented 
on that router

it is highly recomneded that you should confihure staic 
routing on that router dirctly conected to isp router

thansk 
help it will help you both