EIGRP Metric Calculator (Bellman-Ford Algorithm)


EIGRP Metric Calculator

Calculate the composite metric for Cisco’s EIGRP, which uses principles from the Bellman-Ford algorithm, based on path bandwidth and delay.



Enter the slowest link’s bandwidth along the path, in Kilobits per second (Kbps).


Enter the sum of delays for all outgoing interfaces along the path, in microseconds (μs).






Default values (K1=1, K3=1) consider only bandwidth and delay. EIGRP neighbors must have matching K-values.


Calculated EIGRP Composite Metric

281600
Scaled BW: 1000
Scaled Delay: 100

Chart: Contribution of Bandwidth and Delay to the Metric.

What is the EIGRP Metric Calculation?

The EIGRP metric calculation is how the Enhanced Interior Gateway Routing Protocol determines the “cost” or “distance” to a destination network. While EIGRP is an advanced distance-vector protocol, its core path selection relies on the Diffusing Update Algorithm (DUAL), which guarantees loop-free paths by leveraging principles from the classical Bellman-Ford algorithm. The metric itself is a composite value calculated from several path attributes, allowing for a more nuanced and accurate best-path selection than simpler metrics like hop count.

By default, EIGRP uses only bandwidth and delay to compute its metric, but it can be configured to include path load and reliability. This flexibility makes it highly adaptable. The final metric helps a router choose the best loop-free path among multiple available routes to a single destination. A lower metric value indicates a more preferred path. For those interested in protocol comparisons, understanding the OSPF vs EIGRP differences is a great next step.

EIGRP Metric Formula and Explanation

EIGRP uses a complex formula to combine its various components. The K-values act as toggles or multipliers for these components.

The complete formula is:

Metric = 256 * [ (K1 * Scaled_BW) + (K2 * Scaled_BW / (256 – Load)) + (K3 * Scaled_DLY) ] * [ K5 / (Reliability + K4) ]

However, since the default K-values are K1=1, K3=1, and K2=K4=K5=0, the formula simplifies significantly for most real-world networks:

Default Metric = 256 * (Scaled_BW + Scaled_DLY)

This simplified version is what our calculator uses by default, but you can adjust all K-values to see their impact. The final multiplication by 256 is for backward compatibility with IGRP, EIGRP’s predecessor.

Formula Variables

Variable Meaning Unit / Value Typical Range
Scaled_BW The inverse of the minimum bandwidth on the path, scaled for the formula. Calculated as (107 / Minimum BW in Kbps). Unitless Integer 1 – 10,000,000
Scaled_DLY The cumulative delay of all outgoing interfaces on the path. Calculated as (Total Delay in μs / 10). Unitless Integer 1 – Infinity
K1-K5 Constants used to weight the components of the metric. Unitless Integer 0 – 255
Load/Reliability Optional metrics representing path utilization and stability. Not used by default. Unitless Integer 1 – 255
Table 1: Variables in the EIGRP Metric Calculation.

Practical Examples

Example 1: A Standard Fast Ethernet Path

Imagine a path to a destination network where the slowest link is a single Fast Ethernet connection (100,000 Kbps). The cumulative delay along this path is 110 microseconds.

  • Inputs: Minimum Bandwidth = 100,000 Kbps, Cumulative Delay = 110 μs. (K-values at default)
  • Calculation:
    • Scaled BW = 10,000,000 / 100,000 = 100
    • Scaled Delay = 110 / 10 = 11
    • Metric = 256 * (100 + 11) = 256 * 111 = 28416
  • Result: The calculated EIGRP metric is 28416.

Example 2: A Slower, Multi-Hop Path

Now consider a different path to the same destination. This path involves a slower serial link with a bandwidth of 1,544 Kbps, and due to multiple hops, the cumulative delay is 25,000 microseconds.

  • Inputs: Minimum Bandwidth = 1,544 Kbps, Cumulative Delay = 25,000 μs. (K-values at default)
  • Calculation:
    • Scaled BW = 10,000,000 / 1,544 ≈ 6476
    • Scaled Delay = 25,000 / 10 = 2500
    • Metric = 256 * (6476 + 2500) = 256 * 8976 = 2297856
  • Result: The calculated EIGRP metric is 2,297,856. EIGRP would strongly prefer the first path. Exploring different routing protocols can show how other systems handle this.

How to Use This EIGRP Metric Calculator

Using this calculator is straightforward for any network professional. Here’s a step-by-step guide:

  1. Enter Minimum Path Bandwidth: In the first field, input the bandwidth of the slowest link in the path to your destination. This value must be in Kbps. For example, a 100 Mbps link is 100,000 Kbps.
  2. Enter Cumulative Delay: In the second field, input the sum of all interface delays along the path. This value must be in microseconds (μs).
  3. Adjust K-Values (Optional): By default, the calculator uses K1=1 and K3=1, focusing only on bandwidth and delay. You can change these values to see how they affect the metric. Remember, for routers to become neighbors, their K-values must match.
  4. Interpret the Results: The calculator instantly provides the final EIGRP composite metric. The intermediate values show the scaled bandwidth and delay components, helping you understand their individual contributions. The bar chart provides a visual comparison. This is a core part of configuring EIGRP correctly.

Key Factors That Affect the EIGRP Metric

Several factors influence the final metric. Understanding them is key to network design and troubleshooting.

  • Minimum Bandwidth: This is often the most significant factor. Because the formula uses the inverse of bandwidth, a lower bandwidth value results in a much higher metric. This is why EIGRP strongly avoids slow links.
  • Cumulative Delay: This is an additive value that represents the total latency across a path. It acts as a tie-breaker or secondary influence but can become significant on paths with many hops, even if the bandwidth is high.
  • K-Values: These are the most powerful tool for influencing the metric. By changing the K-values away from the default, you can make EIGRP prioritize delay over bandwidth or even factor in link load and reliability. This is an advanced technique and should be used with caution.
  • Network Topology: The number of hops directly impacts the cumulative delay. A path with more routers will naturally accumulate a higher delay value.
  • Interface Configuration: The bandwidth and delay values used in the calculation are taken from the router’s interface configuration. If these are not set correctly (especially on serial links), the metric calculation will be inaccurate.
  • EIGRP “Wide Metrics”: For modern high-speed links (over 1 Gbps), Cisco introduced “wide metrics” which use a 64-bit calculation for greater precision. This calculator uses the classic 32-bit formula, which is still widely used and fundamental to understanding the protocol’s behavior. An overview of network routing basics can provide more context.

Frequently Asked Questions (FAQ)

1. Why is the Bellman-Ford algorithm mentioned with EIGRP?
EIGRP uses the Diffusing Update Algorithm (DUAL) to ensure loop-freedom. DUAL is a network discovery and path computation algorithm that builds upon the foundational principles of the classical Bellman-Ford algorithm to find the lowest-cost path in a network.
2. What are the default K-values for EIGRP?
The default K-values are K1=1, K2=0, K3=1, K4=0, K5=0. This means that by default, only bandwidth (K1) and delay (K3) are considered in the metric calculation.
3. Why do K-values need to match between EIGRP neighbors?
K-values are a fundamental part of how a router perceives network “cost.” If two routers have different K-values, they will calculate metrics differently and disagree on which paths are best, preventing them from forming a stable neighbor relationship.
4. Why is bandwidth inverted in the formula (10^7 / BW)?
The metric needs to be a “cost,” where lower is better. Since higher bandwidth is better, its value needs to be inverted so that a high-bandwidth link results in a low-cost component for the final metric.
5. What units does the calculator use?
The calculator requires bandwidth in Kilobits per second (Kbps) and delay in microseconds (μs). These are the standard units used in Cisco IOS for EIGRP calculations.
6. Should I use Load (K2) and Reliability (K4, K5) in my calculation?
It is generally not recommended. Load and reliability are values that can change frequently, which could cause EIGRP to constantly recalculate paths, leading to network instability. Most network designs rely solely on the stable values of bandwidth and delay.
7. What is a “feasible distance” in EIGRP?
Feasible Distance (FD) is the term EIGRP uses for the total calculated metric of the best path to a destination. This is the value our calculator computes.
8. How does this relate to IGRP?
EIGRP is the successor to IGRP. The EIGRP metric formula is based on the IGRP formula but is multiplied by 256 to scale the metric from 24 bits to 32 bits, allowing for more granularity. This makes an IGRP vs EIGRP comparison a topic of historical interest.

Related Tools and Internal Resources

Expand your knowledge of networking concepts with these related articles and tools.

Disclaimer: This calculator is for educational purposes and should be used as a guide. Always verify configurations in a lab environment before deploying to production.


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