Delay command on Cisco routers

Delay is the time a data packet takes to reach the destination. It is measured in fractions of a second. Usually, it is calculated between the source and the destination. But if the path involves multiple devices, you can calculate each device's delay separately. There are many types of delay. Each type measures the delay of a particular device along the path or of a specific stage in the transmission.

The following table lists the most common types of delay.

Uses and Purposes of the delay command

Cisco routers include the delay command. The delay command configures the interface delay in the running configuration. Many services and protocols running on the router use the interface delay for various purposes. For example, the EIGRP routing protocol uses it to calculate a path's metric. If you change the interface delay in the running configuration, the protocols will use the new delay. You can use the delay command to influence the protocols that use the interface delay. A delay is the physical property of the transmission. The delay command does not change the interface's physical delay. It allows you to configure a new delay for the interface in the running configuration.

Using the delay command to influence the EIGRP metric

To select the best path to a destination, EIGRP calculates and compares the metrics of all available paths to that destination. EIGRP uses interface delay as a metric component. If you modify the delay on an interface, EIGRP will recalculate the metrics of the paths associated with that interface. By changing the interface delay, you can force EIGRP to select a specific route to a particular destination.

The following image shows a network. In this network, Router0 has two routes to reach the network 50.0.0.0/8.

Route1: - via Router2
Route2: - via Router1 and Router2

You can download this lab from the following link.

Packet Tracer Lab for the delay command

Viewing the default delay

Cisco assigns a default delay to each interface. The default delay of a serial interface is 2000. If you do not change it, protocols will use it as it is. If you change it, protocols will use the modified delay. The "show interface [interface name]" command displays the interface delay. The output of this command shows the delay in the tens of microseconds (usec). For example, if the configured delay is 2000, then the output of this command will show the delay 20000 (2000*10 = 20000).

Forcing EIGRP to use a custom route

With the default configuration, EIGRP selects Route1 to reach the network 50.0.0.0/8. To view EIGRP routes, use the "show ip route eigrp" command.

Suppose you want EIGRP to select Route2 to reach the network 50.0.0.0/8 without changing the network's physical layout. In this situation, you can use the delay command to force EIGRP to select Route2. Configure a higher delay on the Serial 0/0/1 interface than the total delay of Route2.

Changing the default delay of an interface

The delay command in interface configuration mode changes the interface’s delay. For example, the following command configures the delay on the Serial 0/0/1 interface to 8000.

Router(config)#interface serial 0/0/1
Router(config-if)#delay 8000

This change will force EIGRP to recalculate the metrics for all paths associated with the Serial 0/0/1 interface. As the delay on the 0/0/1 serial interface increases, the metric for all paths on that interface will increase as well. If EIGRP has multiple paths to a destination, it selects the path with the lowest metric. After recalculation, the metric of Route1 (Via - Serial 0/0/1 interface) will exceed the metric of Route2 (Via - Serial 0/0/0). This change will force EIGRP to choose Route2 to reach the network 50.0.0.0/8.

In the Routing table, EIGRP inserts only the route that has the lowest metric. To view all available routes and their metrics, use the "show ip eigrp topology" command. This command prints all entries from the Topology table. EIGRP uses the Topology table to store information about all available routes for each destination.

Conclusion

The delay command allows you to influence the routing decisions of protocols such as EIGRP, which use delay as the metric. By understanding and configuring interface delay, you can optimize path selection, troubleshoot routing issues, and ensure more efficient use of network resources. Proper use of the delay command lets you control how traffic flows through the network.