How to configure Default Routing in Cisco Routers

When a packet arrives at a router’s interface, the router examines the destination network address and searches its routing table for a matching entry. If a match is found, the router forwards the packet using the specified interface. If no match is found, it checks for a default route. If a default route exists, the router forwards the packet; otherwise, it is dropped. The default route serves as a catch-all path for packets whose destination addresses are not listed in the routing table.

You can use a default route in the following situations.

• To forward all packets to a single destination.
• To forward all unknown packets (whose destination network addresses are not available in the routing table) to a server or a device for logging and troubleshooting purposes.
• To forward all packets to a device or a router that knows how to reach remote networks or has routing table entries for them.

This tutorial explains how to configure a default route to forward all packets to a single destination.

Configure default routing for a single exit point

If two sites are connected through a single link, configuring a default route is more efficient than using a routing protocol, which consumes bandwidth through broadcast messages. A default route enables communication between connected devices without additional bandwidth overhead. For example, if a company’s central office connects to multiple remote sites, and each remote site has only one route to the central office, default routing is the optimal solution.

Lab setup

This lab is optional. If you only need the commands and configuration steps for default routing, feel free to skip ahead. However, if you want hands-on practice or are preparing for a certification, completing this lab will deepen your understanding. You can use any network simulator or real devices. This tutorial uses Packet Tracer. The lab is available at the following link.

Packet Tracer practice lab with essential configuration

Default route commands and configurations

The following command in Global configuration mode configures a default route. Run this command on all remote site routes (RS1, RS2, RS3, and RS4).

Router(config)#ip route 0.0.0.0 0.0.0.0 [exit-interface or IP address of the next-hop]

To use the exit-interface, specify the name of the local interface connected to the central office.
To use the IP address of the next-hop in the configuration, specify the IP address of the interface on the central office’s router connected to this (local) router.

Configure a default route on RS1 and RS2 using the exit-interface option.

Remote site 1 (RS1)

Router(config)#ip route 0.0.0.0 0.0.0.0 serial 0/0/0

Remote site 2 (RS2)

Router(config)#ip route 0.0.0.0 0.0.0.0 serial 0/0/0

Since both routers, RS1 and RS2, are connected to the central office via their serial 0/0/0 interfaces, specify serial 0/0/0 in the exit-interface option of both commands.

Configure a default route on RS1 and RS2 using the IP address of the next-hop option.

Remote site 3 (RS3)

Router(config)#ip route 0.0.0.0 0.0.0.0 30.0.0.1

Remote site 4 (RS4)

Router(config)#ip route 0.0.0.0 0.0.0.0 40.0.0.1

Remote sites RS3 and RS4 are connected to the central office’s router via the serial 0/1/0 (30.0.0.1) and 0/1/1 (40.0.0.1) interfaces, respectively. To configure default routes on these routers, specify the IP addresses configured on these interfaces as the next-hop options of both commands.

Enabling routing

When a packet arrives at a router's interface, the router looks up the destination network of the incoming packet in its routing table. If the routing table contains an entry for the destination network, the router uses that entry to forward the packet. If not, it uses the default route to forward the packet. Since you did not configure any other routes on any remote site, all remote sites will use their default routes to forward all incoming packets.

After this configuration, all remote sites can forward their data packets to the central office, but they will not receive any return packets from the central office, as the central office router has no routing entry in its routing table. To verify this, send ping requests from the PC at the remote site RS1 to the central office server.

Since the central office’s router has multiple exit points, you cannot use a default route on this router. You also cannot use a routing protocol on this router, as routing protocols rely on routing broadcasts from neighboring routers, and you did not configure one on any remote site. In this situation, you can configure the static routing on the central office’s router.

Router(config)#ip route 60.0.0.0 255.0.0.0 10.0.0.2
Router(config)#ip route 70.0.0.0 255.0.0.0 20.0.0.2
Router(config)#ip route 80.0.0.0 255.0.0.0 30.0.0.2
Router(config)#ip route 90.0.0.0 255.0.0.0 40.0.0.2

Once the static routing has been configured on the central office’s router, all remote sites can exchange data with the central office’s network as well as communicate with each other through the central office’s router. To verify this, send ping requests from the PC at the remote site RS1 to the PCs of other remote sites.

The following link provides this lab with the above configuration.

Packet Tracer lab with the default route configuration

Conclusion

Default routing is an essential concept in network design. By configuring a default route, you can ensure that packets destined for unknown networks are forwarded to a predetermined gateway, reducing the complexity of the routing table and minimizing administrative overhead. This tutorial demonstrated how to implement default routing on Cisco routers using both exit-interface and next-hop IP address options. It also highlighted scenarios where default routing is advantageous and explained its limitations, particularly in networks with multiple exit points.