i've tried "route add default gw 172.21.0.1 ppp0", after which the routing table looks thus.Your problem is that you aren't defining a gateway address.
Kernel IP routing tableDestination Gateway Genmask Flags MSS Window irtt Iface 172.21.0.1 0.0.0.0 255.255.255.255 UH 0 0 0 ppp0 192.168.0.0 0.0.0.0 255.255.255.0 U 0 0 0 eth0 0.0.0.0 172.21.0.1 0.0.0.0 UG 0 0 0 ppp0 0.0.0.0 0.0.0.0 0.0.0.0 U 0 0 0 ppp0
but i still can't ping 172.21.0.1 from a client.