Default Routes in Cisco Routers Explained

A routing table contains all the routes the router knows. A router uses it to make forwarding decisions. When a router receives a data packet, it finds a route to the packet's destination network in its routing table. If the routing table contains a route to the packet's destination network, it uses that route to forward the packet. If not, it looks for a default route. If the routing table has a default route, it uses that to forward the packet. A default route allows the router to forward packets for networks for which the routing table has no entry. If neither a valid route nor a default route is available, it discards the packet.

Default route addresses

A default route uses an address that contains all zeros.

IPv4 default route: - 0.0.0.0 0.0.0.0
IPv6 default route: - ::/0

An address consisting only of zeros represents all addresses. In other words, a default route address belongs to all addresses. It allows a router to forward all packets from the default route regardless of their destination addresses.

Why do routers not use the default route in the first place?

A router checks the routing table entries for each packet from top to bottom until it finds a match. Once a match is found, it does not check the following entry for that packet. Since a default route matches all addresses, using it first forces the router to forward all packets through it.

A router uses a default route to forward a packet only when no other route for the packet's destination network is available in the routing table. If a route to a destination network is available, the router never uses the default route for that network.

The default route command

To configure an IPv4 default route, use the following syntax from the global configuration mode.

Router(config)# ip route 0.0.0.0 0.0.0.0 [Next_Hop_IP | Exit_Interface]

To configure an IPv6 default route, use the following syntax from the global configuration mode.

Router(config)# ipv6 route ::/0 [Next_Hop_IP | Exit_Interface]

In both syntaxes:-

• The Next_Hop_IP is the IP address of the device on the other side of the default route.
• The Exit_Interface is a local interface of the router.

Using the Exit_Interface over the Next_Hop_IP has an advantage. The Next_Hop_IP option depends on an IP address configured on a remote device. If the remote device changes it, the default route on this device will not work. Unlike the Next_Hop_IP option, the Exit_Interface does not rely on the remote device.

Examples

The following command configures an IPv4 default route that forwards all packets from the serial 0/0/0 interface.

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

The following command configures an IPv4 default route that forwards all packets to the next hop 192.168.1.1.

Router(config)# ip route 0.0.0.0 0.0.0.0 192.168.1.1

The following command configures an IPv6 default route that forwards all packets from the serial 1/1/1 interface.

Router(config)# ipv6 route ::/0 serial 0/0/0

The following command configures an IPv6 default route that forwards all packets to the next hop 2001:DB8:1:F::1.

Router(config)# ipv6 route ::/0 2001:DB8:1:F::1

To view the default route configuration, you can use the "show ip route" or "show running-config" commands in Privilege EXEC mode.

Router#show ip route
Router#show running-config

Static default route v/s Dynamic default route

A default route configured by the "ip route" command is called the default static route. Some routing protocols, such as RIP and EIGRP, allow you to advertise the default static route. A default static route configured on another router and learned via a routing protocol is called a dynamic default route.

AD (Administrative Distance) value of the default route

AD (Administrative Distance) is the trustworthiness of the source that adds routing information to the routing table. It assigns a unique number to each source from 0 to 255. In this range, a lower number is more trustworthy than a higher one. Routers use the AD value to select the best route from all available routes. If a router receives routing updates for a single network from multiple sources, it uses the AD value to choose a single best route from all available routes.

The AD value of the default route depends on the configuration type. The default AD value of a static default route is 1. The default AD value of a dynamic default route is the AD value of the routing protocol that advertises it. For example, if RIP advertises a default route, its AD value will be 120. The AD value of the RIP routing protocol is 120.

Example (how a default route works in routing)

A packet arrives on an interface of a router. The router reads the destination network from the packet and looks it up in the routing table. There are three possible results of the lookup. The following table describes each situation and the action the router will take in that situation.

Conclusion

A default route is a fallback when specific routes are unavailable. It ensures the router does not unnecessarily discard packets. Understanding its configuration, types, and AD value helps you manage routing effectively.