This tutorial explains how to view and configure interface delay on Cisco routers and use the delay command to influence the EIGRP metric.
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 the delay of each device separately.
There are many types of a delay. Each type measures the delay of a particular device used in the path or the delay of a specific stage in the transmission. The most common types of delay are the following.
Transmission delay
It is the time the source takes to put the data packet on the transmission medium or the link.
Propagation delay
It is the time the transmission medium or the link takes to transfer the data packet.
Queueing delay
A destination device or a router can process only one data packet at a time. If it receives more than one packet, it puts them in the queue. Queuing delay is the time the data packet spends in the queue.
Processing delay
It is the time the destination device or the router takes to process the data packet.
Interface delay
A router interface organizes all the data packets into a serial queue and loads them one by one onto the connected medium. An interface delay is the amount of time a data packet spends in the serial queue.
Uses and Purposes of the delay command
Cisco routers include the delay command. The delay command allows an administrator to configure 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 the metric of a path. If you change the interface delay in the running configuration, the protocols that read the interface delay from the running configuration will use the modified delay. Administrators 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 physical delay of the interface. It allows you to configure a new delay of the interface in the running configuration.Using the delay command to influence the EIGRP metric
To select the best path for a destination, EIGRP calculates and compares the metrics of all available paths for the destination. EIGRP uses interface delay as a metric component. If you modify the delay of an interface, EIGRP will recalculate the metric of the paths associated with the interface.
By changing the interface delay, you can force EIGRP to select a specific route to a particular destination. Let's understand this process through an example.
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 the practice lab of this network from the following link.
Packet Tracer Lab for the delay command
Since many services and protocols depend on the interface delay, Cisco assigns a default delay to each interface. The default delay of a serial interface is 2000. If you don't change the default delay, protocols will use the default delay. If you change the default delay, protocols will use the modified delay.
To view the delay of an interface, you can use the "show interface [interface name]" command. 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).
The following image shows how to use the "show interface" command on Router0 to view the delay of both serial interfaces.
With the default configuration, EIGRP selects Route1 to reach the network 50.0.0.0/8. To view EIGRP routes, you can use the "show ip route eigrp" command. The following image shows the output of this command on Router0.
Now suppose, for some reason, we want EIGRP to pick Route2 to reach the network 50.0.0.0/8. But, at the same time, we don't want to make any change in the physical layout of the network.
In this situation, we can use the delay command to force EIGRP to select Route2. We can configure the delay on the Serial 0/0/1 interface higher than the total delay of Route2.
To configure the delay on an interface, we use the delay command in the interface mode of the interface. For example, the following commands configure the delay of the Serial 0/0/1 interface to 8000.
Router(config)#interface serial 0/0/1 Router(config-if)#delay 8000
The following image shows how to use the above commands on the router.
This change will force EIGRP to recalculate the metric of all paths related to the Serial 0/0/1 interface. As we have increased the delay of the serial 0/0/1 interface, the metric of all paths belonging to the serial 0/0/1 interface will also increase.
To learn how EIGRP metric calculation formula works or how EIGRP calculates the metric of each route, you can check the following tutorial.
EIGRP Metric K Values Explained with Examples
If EIGRP has more than one path to a destination, it selects the path that has 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.
The following image show this exercise step-by-step.
In the Routing table, EIGRP inserts only the route that has the lowest metric. To view all available routes and their metrics, you can 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.
The following image shows the output of this command.
That's all for this tutorial. In this tutorial, we discussed what the delay is and how to use the delay command to influence the EIGRP metric.