Connected Routes and Local Routes Explained

Routers automatically extract two types of routing information from the IP configuration you assign to their interfaces. These types are Connected (code C) and Local (code L) routes. If an interface’s state or IP configuration changes, it automatically updates related Connected (code C) and Local (code L) routes in the routing table. This tutorial explains this process and these route types in detail with examples.

How a router adds and manages Connected and Local routes

You assign an IP address to a router’s interface and enable it. The router checks Line and Protocol status of the interface. Line status indicates layer-2 connectivity. Protocol status shows layer-3 configuration. If Line status is UP, the interface is connected to the remote device. If Protocol status is UP, the interface is enabled and has a valid IP configuration.

• An interface can forward packets only if both status are UP. If either status is Down, it cannot forward packets.
• Routers use the routing table to forward packets. A router does not add routing information for an interface that cannot forward packets.
• If both status are UP, the router takes the following steps.
  • Pick the IP address of the interface.
  • Find the network address of the IP address.
  • Create a routing table entry that includes the network address and the interface name.
  • Add the entry to the routing table.
• If you change the interface’s IP address, the router automatically updates the routing table entry to reflect the change.
• A router cannot forward packets over a disabled interface. As a result, if you disable an interface, the router automatically removes the corresponding routing table entry.

Packet Tracer Example

Packet Tracer is a network simulator software. It allows you to configure and use essential Cisco devices in a simulated environment. Open Packet Tracer and add a Router and PC to the workspace.

Access the Router's CLI prompt.

By default, a router does not add any routing information to the routing table. You can verify it by listing the routing table entries. The show ip route command in Privilege Exec mode shows routing table entries.

Router>
Router#show ip route

Assign the IP address 10.0.0.1/8 to the GigabitEthernet0/0 interface and enable it.

Router#configure terminal
Router(config)#interface GigabitEthernet0/0
Router(config-if)#ip address 10.0.0.1 255.0.0.0
Router(config-if)#no shutdown
Router(config)#

When you enable an interface, the router examines its IP configuration and prepares the corresponding routing information. However, it does not add this information to the routing table until both the Line and Protocol status of the interface turn UP. You can confirm this by using the 'show ip route' command before and after both status are UP. Initially, the output will not display any Connected (C) or Local (L) routes for the interface, but once both status are UP, the routing table output will show the newly added entries. It verifies that a router adds routes only for fully operational interfaces.

Router(config)#exit
Router#show ip route

To check the Line and Protocol status, use the 'show interfaces interface_name' command. The first line of the output displays the interface's Line and Protocol status.

As the above output shows, the interface’s protocol status is UP, but its Line status is Down. Line status will turn UP only when you connect this interface to an active remote device. Connect PC0 to GigabitEthernet0/0.

List routing table entries again.

#show ip route

As soon as both status turn UP, the router automatically adds the routing information for the interface. The router adds two routes for the interface. The router assigns the code letter C to the first route and letter L to the second route. The router uses the first route to forward packets from the interface and the second route to send packets to the interface.

Connected (C) route

A router uses interfaces to receive and forward packets. An interface and the remote device connected to it can exchange packets only if they are from the same IP subnet. Because of this, when you configure an IP address on a router interface, the router automatically assumes that the remote device connected to that interface belongs to the same IP subnet.

In this example, you configured the IP address 10.0.0.1/8 on GigabitEthernet0/0. From this IP address, the router assumes that the remote device on the other side of GigabitEthernet0/0 is on the 10.0.0.0/8 network and knows how to access it. Based on this assumption, the router automatically adds a route for the network 10.0.0.0/8 and associates it with the GigabitEthernet0/0 interface. The router uses the letter C (Connected) to represent this type of route.

When the router receives a packet for a remote destination, it checks all available routes. If a route to the remote destination exists, the router forwards the packet out the interface specified by the route. In this example, if the router receives a packet whose destination address is in the 10.0.0.0/8 network, the router forwards it from the GigabitEthernet0/0 interface.

Local (L) route

A router uses local routes to manage packets that are destined for its own interfaces. You can use these routes to access the router using remote management protocols such as Telnet and SSH. These routes are also called host routes. The router denotes these routes with the letter L (Local), indicating that packets matching this destination should be processed internally by the router rather than forwarded.

In this example, the IP address 10.0.0.1/8 is configured on the GigabitEthernet0/0 interface. To forward packets to the GigabitEthernet0/0 interface, the router adds a local route for this IP address. A local route uses the interface’s IP address with a /32 subnet mask. The prefix /32 represents a host address.

Similarities between a connected route and its related local route

• Both routes belong to the same IP subnet.
• The router calculates both routes from the same IP configuration.
• Both routes lead packets to the same interface.

Differences between a connected route and its related local route

• A connected route represents the network address. It uses the actual subnet prefix (mask). A local route represents the host address. It always uses the subnet prefix /32.
• A router always uses a connected route to forward packets out of the router. A router never uses a local route to send packets out of the router.
• Local routes display the router's internal layout. Connected routes show how the router is connected to the network.

Managing connected routes and local routes

A router creates a connected route and the local route from the IP configuration of the interface. If you change the IP configuration of the interface, the router automatically updates its connected route and the local route.

As mentioned earlier, a router adds a route for an interface only if both the Line and Protocol status are UP. If either or both change to Down, the router automatically removes the interface's route.

If an interface loses its connectivity to the remote device, the router automatically removes its routes. To verify it, remove the cable, and list the routing table entries again.

Conclusion

Understanding how routers add and manage Connected (C) and Local (L) routes is essential for effective network configuration and troubleshooting. A router dynamically updates its routing table based on the status and IP configuration of its interfaces, ensuring that only operational interfaces contribute routes. Connected routes allow routers to forward packets to remote networks, while local routes enable direct communication with the router's own interfaces. By monitoring interface status and routing table entries, you can efficiently maintain connectivity and quickly identify network issues.