CCNA EIGRP Metric: K-Values and Bandwidth Calculation

EIGRP (Enhanced Interior Gateway Routing Protocol) uses a composite metric to calculate the best path to a destination. Unlike simpler protocols such as RIP, which rely on hop count alone, EIGRP combines multiple factors using K-values and weighted components. Understanding the EIGRP metric formula and how K-values influence route selection is essential for CCNA candidates and network engineers responsible for EIGRP deployments.

The EIGRP Metric Formula

EIGRP calculates the composite metric using this formula:

Metric = [K1 * Bandwidth + (K2 * Bandwidth) / (256 - Load) + K3 * Delay] * [K5 / (Reliability + K4)]

Where:
- K1, K2, K3, K4, K5 are weighting constants
- Bandwidth is the minimum bandwidth along the path (in kbps)
- Delay is the sum of all interface delays along the path (in tens of microseconds)
- Load is the interface load (0-255)
- Reliability is the interface reliability (0-255)

By default, K1 = 1, K2 = 0, K3 = 1, K4 = 0, and K5 = 0. This simplifies the formula to:

Metric = Bandwidth + Delay

In practice, EIGRP primarily uses bandwidth and delay. Load and reliability are rarely modified in production environments because they can cause route flapping and instability.

Understanding K-Values

K-values are weighting coefficients that determine how much each metric component contributes to the composite metric. They allow network engineers to tune EIGRP behavior for specific network requirements.

• →K1: Weights the bandwidth component. Default is 1.
• →K2: Weights the load component. Default is 0 (disabled).
• →K3: Weights the delay component. Default is 1.
• →K4: Weights the reliability component. Default is 0 (disabled).
• →K5: Used in the reliability calculation. Default is 0 (disabled).

Changing K-values requires all routers in the EIGRP autonomous system to use identical values. If K-values differ, routers will not form neighbor adjacencies. Configure K-values using the metric weights command:

router eigrp 100
 metric weights 0 1 0 1 0 0

Bandwidth Calculation in EIGRP

EIGRP uses the minimum bandwidth (bottleneck bandwidth) along a path to calculate the metric. The bandwidth value is derived from the interface bandwidth configuration, not actual link utilization. By default, Cisco routers assign bandwidth based on interface type, but you can manually set it.

EIGRP internally represents bandwidth as 10,000,000 divided by the configured bandwidth in kbps. For example, a 1 Gbps link has a bandwidth metric of 10,000,000 divided by 1,000,000 = 10. A 100 Mbps link has a metric of 10,000,000 divided by 100,000 = 100.

interface GigabitEthernet0/0
 bandwidth 1000000
 ! Sets bandwidth to 1 Gbps (1,000,000 kbps)

Delay is summed across all interfaces in the path. Each interface has a default delay value (for example, Gigabit Ethernet defaults to 10 microseconds). The total delay is expressed in tens of microseconds.

Practical Considerations

• →Always verify bandwidth settings match your actual link speeds to ensure accurate metric calculations.
• →Use the show interface command to confirm configured bandwidth and delay values.
• →Modifying K-values requires a network-wide change and should be thoroughly tested before deployment.
• →EIGRP metric values can grow very large; use the show ip eigrp topology command to inspect feasible distance and reported distance.
• →Bandwidth is a logical parameter; changing it does not affect throughput but only influences route selection.

Mastering EIGRP metrics prepares you for real-world network design and troubleshooting. When routes behave unexpectedly, incorrect bandwidth or K-value configuration is often the culprit. Verify metrics regularly and document your settings for consistency across your network.