Default route

Default route
A default route is similar to a static route, but instead of configuring a route to a specific network, you are configuring the router to know where to send traffic for any network not found in its routing table. Default routes are used to establish a gateway of last resort for your router.

Router(config)# ip route 0.0.0.0 0.0.0.0 IP_address_of_next_hop_neighbor [administrative_distance] [permanent]
Or
Router(config)# ip route 0.0.0.0 0.0.0.0 interface_to_exit [administrative_distance] [permanent]

permanent (Optional) Specifies that the route will not be removed, even if the interface shuts down.

RTA(config)# ip route 0.0.0.0 0.0.0.0 172.16.3.2
Or
RTA(config)#ip route 0.0.0.0 0.0.0.0 s0/0

RTB(config)#ip route 0.0.0.0 0.0.0.0 172.16.3.1
Or
RTB(config)#ip route 0.0.0.0 0.0.0.0 s0/0