EIGRP Metric K Values Explained with Examples

EIGRP calculates the cost of all available routes between the source and destination. After calculating the cost of all routes, it selects the best route. It adds the selected route to the routing table. If there is only one route, it selects that route. If multiple routes exist, it selects the route with the least cost. To calculate the cost, a formula is used. This tutorial explains how this formula works and which components it uses to calculate the cost.

EIGRP composite metric calculation formula

EIGRP uses a composite metric calculation formula. The formula can use five components in its calculation. These components are Bandwidth, Delay, Load, Reliability, and MTU. These components can be enabled and disabled individually. The formula includes only enabled components in the calculation. By default, only Bandwidth and Delay are enabled. To include the remaining components, you must enable them manually.

K-values

K-values serve as control variables within the formula. You can use them to enable or disable specific components. A K-value determines whether its associated component is included in the metric calculation. The following table lists all K-values and their associated components.

Bandwidth (K1)

Bandwidth refers to the maximum data transfer rate over a link within a specified time interval. This value remains static unless physical (Layer 1) modifications, such as cable replacement or link upgrades, are implemented. EIGRP selects the lowest bandwidth among all outgoing interfaces along a route. For example, consider a network where Router0 has two possible routes to reach Router5: Route1 and Route2.

EIGRP uses Update messages to share routing information. An EIGRP message includes the value of all metric components. From EIGRP update messages, all EIGRP routers learn the value of all components for each subnet.

From EIGRP update messages, Router0 learn the bandwidth of each link on both routes.
The bandwidths of the exit interfaces on Route1 are 72 Kbps, 28 Kbps, and 28 Kbps. EIGRP will use 28 Kbps to calculate the cost of Route1.
The bandwidths of the exit interfaces on Route2 are 56 Kbps, 56 Kbps, and 64 Kbps. EIGRP will use 56 Kbps to calculate the cost of Route2.

Why does EIGRP select the lowest bandwidth rather than the highest?

Selecting the minimum bandwidth ensures that the available bandwidth along the entire route is at least as high as the lowest segment. This approach guarantees consistent bandwidth throughout the path.

Load represents the volume of traffic traversing an interface relative to its maximum capacity. It is expressed on a scale from 1 (idle) to 255 (fully utilized). It is a dynamic value. It fluctuates based on packet rate and bandwidth. Because data flows bidirectionally, the router maintains two distinct load counters:

• Txload for the outgoing traffic
• Rxload for the incoming traffic

If K2 is enabled, the maximum Txload value will be used in the composite metric calculation formula.

Delay (k3)

Delay measures the time required for a packet to traverse an interface, expressed in tens of microseconds. It is a static value. You can configure it using the 'delay' command. EIGRP uses the total delay in the metric calculation formula. Total delay is the sum of the delay received from the neighboring router and the delay configured on the interface. For example, if EIGRP receives a delay value of 1000 from a neighboring router and the local interface is configured with a delay of 2000, the total delay will be 3000 (1000 + 2000).

To view the configured delay on an interface, use the 'show interface' command. The output of this command displays the value of all components used in the EIGRP metric calculation formula.

Reliability (K4)

Reliability is the ratio of successfully received frames to the total number of received frames. A value of 100% indicates all frames are error-free, while lower values signify the presence of corrupted frames. Reliability is expressed on a scale of 0 to 255. 255 expresses 100% reliability while 0 represents 0% reliability. If K4 is enabled in the metric calculation formula, it will use the minimum reliability.

MTU (K5)

MTU (Maximum Transmission Unit) is included in routing updates but does not directly affect the metric calculation. EIGRP references MTU when the number of equal-cost paths exceeds the permitted limit, discarding the path with the lowest MTU in such cases. EIGRP supports load balancing. Load balancing allows routers to use multiple paths to a single destination. It can utilize up to 32 paths, with a default of 4. The number of permitted paths is configurable via the "maximum-paths" command.

EIGRP Metric Calculation Formula

If all five components are enabled, it uses the following formula to produce a single 32-bit metric.

As mentioned earlier, Load, Reliability, and MTU are disabled by default. If a component is disabled, EIGRP does not use its value in the formula. If the disabled components are excluded, the above formula becomes:

Cisco uses the following default configuration values for Bandwidth and Delay.

Bandwidth = 10⁷/ least bandwidth of the route [Lowest bandwidth from all interfaces between source and destination.]

Delay = cumulative delay of the route [Sum of all outgoing interfaces' delay.]

Example

Create a practice lab as shown in the following example. You can use any network simulation software. This tutorial uses Packet Tracer. The following link provides this lab.

Packet Tracer Lab for EIGRP metric calculation formula

Run the 'show ip route eigrp' command on Router0 to view all routes in the routing table added by EIGRP.

As shown above, EIGRP has added four routes.
The destination subnet of the first route is 30.0.0.0/8.
The destination subnet of the second route is 40.0.0.0/8.
The destination subnet for the third and fourth routes is 50.0.0.0/8. Since the third and fourth routes have the same metric, they are both added for the load balancing.

The value after the forward slash in the square bracket is the route metric. For example, the metric of the first route (30.0.0.0/8) is 2681856.

EIGRP metric calculation example 1

The destination subnet of the first route (30.0.0.0/8) is available between Router1 and Router2. There are two exit points (interfaces) between the source and the destination. These exit points are Router0's S0/0/0 and Router1's S0/0/1. Since you did not change the default values of the bandwidth and delay on both interfaces, EIGRP will use the default values in the formula. The default bandwidth and delay of a serial interface are 1544Kbps and 2000, respectively.

To view the value of all metric components, you can use the 'show interface' command.

The formula uses the least bandwidth from all interfaces between the source and the destination. Since both interfaces have equal bandwidth, the minimum bandwidth is 1544 kbps.

The formula adds the configured delays across all interfaces between the source and the destination. The sum of the delays configured on both interfaces is 4000 (2000 + 2000).

The output of the 'show interface' command shows the delay in tens of microseconds. Because of this, the delay is shown as 20000 (2000 × 10) in the output above.

As you know, unless you enable Load, Reliability, and MTU, EIGRP uses the following formula.

For the above formula, you only need the least bandwidth and cumulative delay. The least bandwidth and cumulative delay of the first route are 1,544 kbps and 4,000, respectively. Let's put both values in the formula.

Metric = ((10000000/1544) + 4000)*256
Metric = ((6476.6839) + 4000)*256

If the result of an operation is in decimal, the formula rounds it back to the nearest integer before performing the rest of the equation. Because of this, the value 6476.6839 will be rounded back to 6476.

Metric = ((6476) + 4000)*256
Metric = (10476)*256
Metric = 10476*256
Metric = 2681856

Thus, the cost of the first route (30.0.0.0/8) is 2681856.

EIGRP metric calculation example 2

Let's calculate the cost of the second route (40.0.0.0/8).

This route has two exit points (interfaces) till the destination subnet. Both exit interfaces have default bandwidth and delay. As a result, the minimum bandwidth and cumulative delay will be 1544 kbps and 4000, respectively.

Let's put these values in the equation.

Metric = ((10000000/1544) + 4000)*256
Metric = ((6476.6839) + 4000)*256
Metric = (6476 + 4000) * 256
Metric = 10476 * 256
Metric = 2681856

Thus, the cost of the second route (40.0.0.0/8) is 2681856.

EIGRP metric calculation example 3

There are two routes to the subnet 50.0.0.0/8. Both routes have the least bandwidth and cumulative delay. Since their least bandwidth and cumulative delay are the same, their cost will also be the same. The equal-cost routes are automatically used for load balancing. As a result, EIGRP added both routes to the routing table.

The following steps calculate the metric for both routes.

Metric = ((10000000/1544) + 4010) * 256
Metric = (6476 + 4010) * 256
Metric = 10486 * 256
Metric = 2684416

Thus, the cost of both routes is 2684416.

Conclusion

EIGRP determines optimal routing paths using a composite metric that incorporates Bandwidth, Delay, Load, Reliability, and MTU. By default, only Bandwidth and Delay are active, but additional components can be enabled for greater control. Understanding the influence of each K-value enables you to optimize routing and enhance data delivery efficiency.