Default routes are used to send traffic to destinations that do not match the routing tables of devices in the network. The most common case for deployment would be the network with Internet access as it would be impossible to contain in the routing tables of all devices that comprise routes.
The default routes, as well as static routes common, are configured using the ip route command in Global Configuration mode.
Router (config) # ip route “0.0.0.0 0.0.0.0″ “IP next hop or output interface”
0.0.0.0 0.0.0.0: last one AND with the destination address and not coincide with the routes in the routing table sends the IP packet to the next hop or output interface.
IP next-hop: The IP of the router interface directly connected to the router where you are configuring the static route.
Interface output: This is the IP of the router where you are configuring the static route. It is used in the case of ignoring the next hop IP.
Default routes - Sample Configuration
For this exercise we will use the same topology we use to configure static routes common to the network by adding an outlet to the Internet.
In this exercise we were required to occur between the networks routing 192.168.1.0, 192.168.2.0 and 192.168.3.0. Now, besides that, we have static routing make from these networks to the Internet. For the exercise, I will use the IP next hop, but also can be done with local output interface.
RouterA: For the 192.168.1.0 network packets are routed to 192.168.2.0 and 192.168.3.0 networks and to the Internet, we configure two static routes to those networks and the default route. In this case for the static routing and default route use the same IP next hop or the same output interface.
RouterA (config) # ip route 192.168.2.0 255.255.255.0 10.0.0.1
RouterA (config) # ip route 192.168.3.0 255.255.255.0 10.0.0.1
RouterA (config) # ip route 0.0.0.0 0.0.0.0 10.0.0.1
RouterB: For the 192.168.2.0 network packets to be routed to 192.168.1.0 and 192.168.3.0 networks, and to the Internet, we set 2 static routes to these networks and the default route. In this case for static IP routing next hop and output interface will be different because the routing is done by different interfaces. For the default route will use the IP of S0 / 1 IP Router C as next hop or S0 / 1 home.
RouterB (config) # ip route 192.168.1.0 255.255.255.0 10.0.0.2
RouterB (config) # ip route 192.168.3.0 255.255.255.0 11.0.0.2
RouterB (config) # ip route 0.0.0.0 0.0.0.0 11.0.0.2
RouterC: For the 192.168.3.0 network packets are routed to 192.168.1.0 and 192.168.2.0 networks and to the Internet, we configure two static routes to these networks and the default route. In this case for static routing IP use the same next hop or the same output interface. To use the default route as the outbound interface S0 / 0 which is the one connected to the Internet and it would be impossible to know what will be the next hop IP.
RouterC (config) # ip route 11.0.0.1 255.255.255.0 192.168.2.0
RouterC (config) # ip route 192.168.1.0 255.255.255.0 11.0.0.1
RouterC (config) # ip route 0.0.0.0 0.0.0.0 S0 / 0
If we want to make settings only for the outgoing interface or we lack any data on the topology, it would be well.
RouterA (config) # ip route 192.168.2.0 255.255.255.0 s0 / 0
RouterA (config) # ip route 192.168.3.0 255.255.255.0 s0 / 0
RouterA (config) # ip route 0.0.0.0 0.0.0.0 s0 / 0
RouterB (config) # ip route 192.168.1.0 255.255.255.0 s0 / 0
RouterB (config) # ip route 192.168.3.0 255.255.255.0 s0 / 1
RouterB (config) # ip route 0.0.0.0 0.0.0.0 s0 / 1
RouterC (config) # ip route 192.168.2.0 255.255.255.0 S0 / 1
RouterC (config) # ip route 192.168.1.0 255.255.255.0 s0 / 1
RouterC (config) # ip route 0.0.0.0 0.0.0.0 S0 / 0
Checking Default Routes
If we know we can check the destination IP default route by using the ping command. In case of failure to show IP route command can display the routing tables. The marked “C” are directly connected networks, those marked with an “S” are the static routes and marked “S *” is the default route.
RouterA # show ip route
Gateway of last resort is 10.0.0.1 to network 0.0.0.0
C 10.0.0.0 / 8 is directly connected, Serial0 / 0
C 192.168.1.0/24 is directly connected, FastEthernet0 / 0
S 192.168.2.0/24 [1 / 0] via 10.0.0.1
S 192.168.3.0/24 [1 / 0] via 10.0.0.1
S * 0.0.0.0 / 0 [1 / 0] via 10.0.0.1
RouterB # show ip route
Gateway of last resort is 11.0.0.2 to network 0.0.0.0
C 10.0.0.0 / 8 is directly connected, Serial0 / 0
C 11.0.0.0 / 8 is directly connected, Serial0 / 1
S 192.168.1.0/24 [1 / 0] via 10.0.0.2
C. 192.168.2.0/24 is directly connected, FastEthernet0 / 0
S 192.168.3.0/24 [1 / 0] via 11.0.0.2
S * 0.0.0.0 / 0 [1 / 0] via 11.0.0.2
RouterC # show ip route
Gateway of Last resort is 0.0.0.0 to network 0.0.0.0
C 11.0.0.0 / 8 is directly connected, Serial0 / 1
C 190.139.83.91/24 is directly connected; Serial0 / 0 (would be the IP Assigned by ISP)
S 192.168.1.0/24 [1 / 0] via 11.0.0.1
S 192.168.2.0/24 [1 / 0] via 11.0.0.1
C 192.168.3.0/24 is directly connected, FastEthernet0 / 0
S * 0.0.0.0 / 0 is directly connected, Serial0 / 0