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.

This tutorial is the last part of the article \”Configure Default Routes in Cisco Router\”. Other parts of this article are the following.

Default Routes in Cisco Routers Explained

This is the first part of the article. This part describes how a default route works as well as also explains the command syntax that is used to configure the default route.

How to configure Default Routing in Cisco Routers

This is the second part of the article. This part explains how to configure default routing in Cisco routers.

Configure a Static Default Route in Cisco Packet Tracer

This is the third part of the article. This part explains how to configure a default route to forward all unknown packets to a logging device for monitoring purposes.

In this part, through a practical example in the Packet tracer, we will understand how to configure, propagate and inject a static default route without changing any setting of the existing network.

LAB Setup

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

practice lab for gateway router

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

configure rip routing

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

test connectivity

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.

adding dsm modem to network

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

Download practice lab with IP configuration

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
Router(config)#router rip
Router(config-router)#default-information originate

The IP address 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.

configure default routing

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.

show ip route command output

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

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

traffic generator

Once the send button is clicked, PC5 continuously sends HTTP packets to the destination IP 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.

http traffic testing

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.

ftp traffic generator setting

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.

ftp traffic to internal network testing

In case if you don’t get a similar output, download the following configured LAB and compare your LAB to detect misconfiguration.

Default routing configured LAB

That’s all for this tutorial. If you like this tutorial, please don’t forget to share it with friends through your favorite social channel.

Prerequisites for 200-301

200-301 is a single exam, consisting of about 120 questions. It covers a wide range of topics, such as routing and switching, security, wireless networking, and even some programming concepts. As with other Cisco certifications, you can take it at any of the Pearson VUE certification centers.

The recommended training program that can be taken at a Cisco academy is called Implementing and Administering Cisco Solutions (CCNA). The successful completion of a training course will get you a training badge.

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