http://journey2ccie.blogspot.com/2010/04/3-method-configuring-default-route-on.html


ip default-network
ip route
ip default-gateway



this example we’ll use RIP, you can also use EIGRP but not OSPF
prepare the network
R1
interface FastEthernet0/0
ip address 10.1.1.1 255.255.255.0
duplex auto
speed auto
!
interface Serial0/0
ip address 192.168.1.1 255.255.255.0
clock rate 64000
!
interface Serial0/1
ip address 172.16.1.1 255.255.255.0
clock rate 64000
!
router rip
network 172.16.0.0
network 192.168.1.0

R2
interface Serial0/0
ip address 192.168.1.2 255.255.255.0
!
router rip
network 172.16.0.0
network 192.168.1.0

R3
interface Serial0/0
ip address 172.16.1.2 255.255.255.0
!
router rip
network 172.16.0.0

make sure R1, R2 and R3 learn all the advertised interfaces and RIP working fine by pinging from one to other interfaces
R1>sh ip rou
R 99.0.0.0/8 [120/1] via 192.168.1.2, 00:00:16, Serial0/0
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.1.0 is directly connected, Serial0/1
10.0.0.0/24 is subnetted, 1 subnets
C 10.1.1.0 is directly connected, FastEthernet0/0
C 192.168.1.0/24 is directly connected, Serial0/0

R2#show ip route
99.0.0.0/24 is subnetted, 1 subnets
C 99.99.99.0 is directly connected, Loopback99
R 172.16.0.0/16 [120/1] via 192.168.1.1, 00:00:15, Serial0/0
C 192.168.1.0/24 is directly connected, Serial0/0
R3#sh ip rou
R 99.0.0.0/8 [120/1] via 172.16.1.1, 00:00:21, Serial0/0
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.1.0 is directly connected, Serial0/0
R 192.168.1.0/24 [120/1] via 172.16.1.1, 00:00:21, Serial0/0

now issue this command on R1
R1(config)#ip default-network 10.0.0.0
*again, remember, this only work on hub spoke network only !!!
now, show ip route on R2 and R3
notice there are default route in the routing table
R2#sh ip route
99.0.0.0/24 is subnetted, 1 subnets
C 99.99.99.0 is directly connected, Loopback99
R 172.16.0.0/16 [120/1] via 192.168.1.1, 00:00:12, Serial0/0
R 10.0.0.0/8 [120/1] via 192.168.1.1, 00:00:12, Serial0/0
C 192.168.1.0/24 is directly connected, Serial0/0
R* 0.0.0.0/0 [120/1] via 192.168.1.1, 00:00:12, Serial0/0
R3#sh ip route
R 99.0.0.0/8 [120/1] via 172.16.1.1, 00:00:17, Serial0/0
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.1.0 is directly connected, Serial0/0
R 10.0.0.0/8 [120/1] via 172.16.1.1, 00:00:17, Serial0/0
R 192.168.1.0/24 [120/1] via 172.16.1.1, 00:00:17, Serial0/0
R* 0.0.0.0/0 [120/1] via 172.16.1.1, 00:00:17, Serial0/0

now we'll configure configure static ip route, using this method we need to redistribute the route to R2 and R3 manually
remove default-network and clear the routing table to make sure it's not gonna influence our new conf in any possible way
on R1
R1(config)#no ip default-network 10.0.0.0
on R1, R2 and R3
R1#clear ip route *

issue the static route command on R1 and specified the exit interface
R1(config)#ip route 0.0.0.0 0.0.0.0 fastEthernet 0/0
then redistribute this static route to RIP
R1(config)#router rip
R1(config-router)#redistribute static
show ip route on R2 and R3 should show the default route now
R3#sh ip route
Gateway of last resort is 172.16.1.1 to network 0.0.0.0
R 99.0.0.0/8 [120/1] via 172.16.1.1, 00:00:15, Serial0/0
172.16.0.0/24 is subnetted, 1 subnets
C 172.16.1.0 is directly connected, Serial0/0
R 10.0.0.0/8 [120/1] via 172.16.1.1, 00:00:15, Serial0/0
R 192.168.1.0/24 [120/1] via 172.16.1.1, 00:00:15, Serial0/0
R* 0.0.0.0/0 [120/1] via 172.16.1.1, 00:00:15, Serial0/0
R2#sh ip route
Gateway of last resort is 192.168.1.1 to network 0.0.0.0
99.0.0.0/24 is subnetted, 1 subnets
C 99.99.99.0 is directly connected, Loopback99
R 172.16.0.0/16 [120/1] via 192.168.1.1, 00:00:01, Serial0/0
R 10.0.0.0/8 [120/1] via 192.168.1.1, 00:00:01, Serial0/0
C 192.168.1.0/24 is directly connected, Serial0/0
R* 0.0.0.0/0 [120/1] via 192.168.1.1, 00:00:01, Serial0/0
this conf also work with EIGRP, but not with OSPF

another way is to use command ip default-gateway, but you need to turn off ip routing to use this
R3(config)#no ip routing
R3(config)#ip default-gateway 172.16.1.1
R3#sh ip route
Default gateway is 172.16.1.1
Host Gateway Last Use Total Uses Interface
ICMP redirect cache is empty
http://journey2ccie.blogspot.com/2010/04/3-method-configuring-default-route-on.html