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.