Default route

Default route:

ip default-gateway [no ip routing]

ip default-network [ip routing]

Static - ip route 0.0.0.0 0.0.0.0 interface nexthop

RIP - default-information originate

EIGRP - redistribute a static route or summarize to 0.0.0.0/0

ip route 0.0.0.0 0.0.0.0 x.x.x.x (next hop to the internet)
!
router eigrp 100
 redistribute static
 default-metric 10000 1 255 1 1500

-----------------------------------------------------------------------------------

router eigrp 100
 network 10.0.0.0
!
interface serial 0.1 point-to-point

frame-relay interface-dlci 10
ip summary-address eigrp 100 0.0.0.0 0.0.0.0


OSPF

There are two ways to inject a default route into a normal OSPFarea.

1. If the ASBR already has the default route in its routing table, you can advertise the existing 0.0.0.0/0 into the OSPF domain with the default-information originate router configuration command.
   
2. If the ASBR doesn't have a default route, you can add the keyword always to the default-information originate command (default-information originate always).
   
   
   ref:How Does OSPF Generate Default Routes?
   OSPF default route: design scenarios
   OSPF Design Guide  

          EIGRP ways to advertise a default route
          Configuring a Gateway of Last Resort Using IP Commands
          rip-default-routes