How Gateway Router Propagates and Injects a Default Route

This tutorial explains how to configure a gateway router to inject a default route in routing through a practical example in the Packet Tracer. Learn how to configure, propagate, and inject a static default route.

When a router receives a packet on its interface, the router reads the destination address of the packet and finds that destination address in the routing table. If an entry for the destination network address is available in the routing table, the router forwards the packet from the interface that is associated with the destination network in the entry.

If an entry for the destination network address is not available in the routing table, the router forwards the packet from the default gateway. In other words, a default route allows a router to forward packets those destination network addresses are not available in the routing table.

A router forwards an incoming packet from the default route only when no route is available for that packet in the routing table. This default behavior allows us to configure a default route for an external network without interfering with the existing routes of the internal network.

LAB Setup

Create a packet tracer LAB as shown in the following image and assign IP configuration.

Configure RIPv2 routing on all three routers (A, B, and C) as shown in the following figure.

To verify the setup, test the connectivity between PC5 and PC1.

Currently, this network is not connected to the Internet. Our objective is to connect this network to the Internet without making any change in existing routes and routing.

Usually, ISPs offer Internet connection through a DSL modem. A DSL modem is the combination of a router and an Ethernet switch. The router of the DSL modem connects the local network to the ISP office. The Ethernet switch of the DSL modem connects the local network to the router of the DSL modem.

To simulate the DSL modem, connect a router on the Ethernet port of the router A and assign IP configuration as shown in the following image.

If you cannot replicate this LAB in your packet tracer, you can download the following pre-configured practice LAB.

Configuring and injecting a default route

Since we have connected the DSL modem (ISP router) to the Fa0/1 Ethernet port of router A, this router will now act as a gateway router. To configure the default routing on this router in such a way that it forwards only internet traffic from the Fa0/1 interface, run the following commands from the global configuration mode.

Router(config)#ip route 0.0.0.0 0.0.0.0 192.168.1.1
Router(config)#router rip
Router(config-router)#default-information originate

The IP address 192.168.1.1 is configured on the Fa0/1 interface of the ISP router and this interface is directly connected to the Fa0/0 interface of the router A. The first command configures a default route to forward packets from the Fa0/0 interface.

The second command changes the global configuration mode into the RIP-routing configuration mode.

The third command instructs the RIP-routing protocol to advertise/propagate the default route through its broadcasts.

To verify that the default route is properly propagated and injected in routing tables of all routers, use the "show ip route" privileged-exec mode command.

As you can see in the above output, the default route has been successfully injected into the routing table of all routers. Now, any packet that is not intended for the internal network will be sent to the gateway router and the gateway router will forward that packet from the Fa0/1 interface to the ISP router.

To verify this setup, access a website that is hosted on the IP 150.0.0.1.

To simulate this scenario, open PC5's traffic generator and make the settings as shown in the following figure and click the Send button.

Once the send button is clicked, PC5 continuously sends HTTP packets to the destination IP 150.0.0.1. If these packets reach the ISP router, our setup will be verified.

To view where all packets reach, click the Simulation button available in the bottom-right corner of the packet tracer. In the Simulation pane, click the Start button.

As you can see in the output above, packets sent to the Internet host have reached the ISP router. This verifies that the network’s PCs can access the Internet through the configured default route.

To verify that we have not messed up existing routes, let's do another test. Open PC5’s traffic generator again and click the Stop button to stop the ongoing testing.

Now, adjust settings as given in the following image, and click the Send button to send the FTP packets to PC1.

If the packets reach PC1, it will confirm that only packets that are intended for an external network will be sent to the ISP router. If packets are intended for an internal device, they will not be forwarded to the ISP router.