After of all ur question is not so clear. But still i m
trying to answer it.

When we use default route - 

a) when our network is a stub network.
b) when we don't know the destination network IP.

In this case, it seems like u don't know the destination
network IP. so we hv to configure Default route on ROUTER 1.

Default route configuration command is - 

Router(config-router)#ip route 0.0.0.0 0.0.0.0 [Next Hop
IP/Exit Interface]

Commands explanation - 

a) ip route: The command used to add a static or default route
b) 0.0.0.0: The wildcard used to say “any” network not
already in the routing table
c) 0.0.0.0: Wildcard mask to say “any” network mask
d) 172.10.0.2/S0(Suppose): Next hop gateway/Exit Interface.



Suppose u don't know the destination network IP, as its
necessary to knw in static routing. How can u configure if u
don't knw this.

Router(config-router)#ip route 0.0.0.0 0.0.0.0 s1

*** (Suppose S1 is the interface which connects to ISP's
router).

So any traffic, that is destined for outside ur network will
be forwarded to S1 interface of ROUTER 1. So whenever ISP's
router receive the packet it'll check the L3 address &
forward accordingly.


Thats all about it. Hope it may help u to understand.

Thank You all.. Keep Posting.

Shahin.

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

Thank u so much Jitendera..!!!!