EIGRP Metric Calculator
Determine the classic composite metric based on the slowest path bandwidth and cumulative delay.
Link Bandwidth
Unit
Link Delay (in microseconds, µs)
Link Bandwidth
Unit
Link Delay (in microseconds, µs)
What is the EIGRP Metric Calculation?
The Enhanced Interior Gateway Routing Protocol (EIGRP) is a sophisticated routing protocol developed by Cisco. To decide the best path to a destination network, it calculates a composite metric. A lower metric value indicates a more preferred path. The core principle to understand is that by default, **EIGRP uses the slowest bandwidth** of any single link along an entire path and the **cumulative (sum of) delay** of all links in that path for its metric calculation. This ensures that the path’s quality is judged by its weakest link, preventing a route with many fast links and one very slow link from being unfairly preferred.
This calculator demonstrates the “classic” EIGRP metric formula. This method is fundamental for network engineers managing and troubleshooting routing behavior in many enterprise networks. For a deeper dive into EIGRP’s mechanics, you might want to explore resources on EIGRP K Values.
The EIGRP Metric Formula and Explanation
By default, EIGRP uses constants known as K-values, where K1 (for bandwidth) and K3 (for delay) are set to 1, and others (K2, K4, K5 for load and reliability) are set to 0. This simplifies the complex formula to one that only considers bandwidth and delay. The simplified formula is:
Metric = [ (107 / Slowest_Bandwidth_kbps) + (Total_Delay_µs / 10) ] * 256
The components are:
- Slowest_Bandwidth_kbps: EIGRP examines the bandwidth of every outgoing interface in the path and selects the single lowest value. This value is measured in kilobits per second (kbps). The term 107 is a reference bandwidth used to scale the value.
- Total_Delay_µs: EIGRP sums the delay values of all outgoing interfaces in the path. This value is measured in microseconds (µs) but is divided by 10 for the calculation, scaling it to tens of microseconds.
- Multiplier (256): This value scales the result to produce the final 32-bit metric value, a legacy feature for backward compatibility with EIGRP’s predecessor, IGRP.
Variables Table
| Variable | Meaning in Formula | Unit | Typical Range |
|---|---|---|---|
| Bandwidth (BW) | The minimum configured bandwidth of any link in the path. | kbps | 1 kbps – 10,000,000 kbps (10 Gbps) |
| Delay (DLY) | The sum of configured delays for all links in the path. | microseconds (µs) | 10 µs – 167,772,150 µs |
Practical Examples
Example 1: Simple Two-Link Path
Imagine a path from a source to a destination across two routers, Link A and Link B.
- Link A: Bandwidth = 100,000 kbps (100 Mbps), Delay = 100 µs
- Link B: Bandwidth = 10,000 kbps (10 Mbps), Delay = 1000 µs
The calculation would be:
- Slowest Bandwidth: The calculator identifies 10,000 kbps as the weakest link.
- Total Delay: The delays are summed: 100 µs + 1000 µs = 1100 µs.
- Formula Application:
Bandwidth Component = 10,000,000 / 10,000 = 1000
Delay Component = 1100 / 10 = 110
Metric = (1000 + 110) * 256 = 284,160
Example 2: Path with a Very Fast Link
Consider a path with a Gigabit link and a slower T1 link.
- Link A (Gigabit): Bandwidth = 1,000,000 kbps (1 Gbps), Delay = 10 µs
- Link B (T1): Bandwidth = 1,544 kbps, Delay = 20,000 µs
Here, the fact that **EIGRP uses the slowest bandwidth** is critical.
- Slowest Bandwidth: 1,544 kbps is the bottleneck.
- Total Delay: 10 µs + 20,000 µs = 20,010 µs.
- Formula Application:
Bandwidth Component = 10,000,000 / 1,544 = 6476
Delay Component = 20,010 / 10 = 2001
Metric = (6476 + 2001) * 256 = 2,170,112
This high metric correctly reflects that the path is constrained by the slow T1 link. For more information on configuring these values, see this guide on EIGRP Configuration.
How to Use This EIGRP Metric Calculator
This tool simplifies the process of finding the EIGRP metric for a given network path.
- Add Links: The calculator starts with two link sections. If your path has more than two links (hops), click the “Add Link” button for each additional link.
- Enter Bandwidth: For each link, enter its bandwidth value and select the correct unit (kbps, Mbps, or Gbps). The tool automatically converts these to kbps for the calculation.
- Enter Delay: For each link, enter its delay in microseconds (µs). This is the value you would see with `show interface` on a Cisco router.
- Calculate and Interpret: The calculator automatically updates the results. The “Total EIGRP Metric” is the primary result. The intermediate values show you exactly how the slowest bandwidth and total delay contributed to the final metric.
Key Factors That Affect the EIGRP Metric
Several factors can influence the final metric. Understanding them is key to network design and troubleshooting.
- Interface Bandwidth: This is the most significant factor on modern networks. The `bandwidth` command on a Cisco interface directly sets this value for EIGRP’s use. It does not change the link’s physical speed but is crucial for routing calculations.
- Interface Delay: The `delay` command sets this value. It’s a simple way to influence path selection without affecting other protocols like QoS that might rely on the bandwidth setting. Increasing delay makes a path less desirable.
- K-Values: While the defaults (K1=1, K3=1) are almost always used, changing them fundamentally alters what EIGRP considers important. For instance, setting K1=0 would make EIGRP ignore bandwidth entirely. Mismatched K-values will prevent routers from becoming neighbors.
- Network Topology: The number of hops directly impacts the cumulative delay. A path with more links will naturally accumulate more delay, increasing its overall metric.
- Summarization: Manual route summarization can obscure the specific metrics of individual paths, potentially leading to suboptimal routing if not carefully planned.
- Wide Metrics: For networks with links faster than 10 Gbps, the classic formula breaks down. EIGRP “Wide Metrics” were introduced to handle these higher speeds with greater precision using 64-bit values. This calculator focuses on the classic 32-bit metric.
Frequently Asked Questions (FAQ)
- 1. Why does EIGRP use the slowest bandwidth and not the average or sum?
- A network path is only as fast as its slowest segment. Using the slowest bandwidth (the “weakest link”) accurately reflects the maximum throughput achievable along that path.
- 2. What is the unit for delay in the EIGRP metric calculation?
- The interface delay is measured in microseconds (µs), but the formula divides it by 10, effectively using units of tens-of-microseconds.
- 3. Can I change bandwidth and delay to influence routing?
- Yes, this is a common traffic engineering technique. You can use the `bandwidth` and `delay` interface commands on a Cisco router to change the values EIGRP uses in its calculation.
- 4. What are the default K-values for EIGRP?
- By default, K1=1 and K3=1. All other K-values (K2, K4, K5) are 0. This means only bandwidth and delay are used in the calculation.
- 5. Do routers need to have matching K-values?
- Yes, absolutely. Two EIGRP routers will not form a neighbor adjacency if their K-values do not match exactly. This prevents routing loops caused by routers evaluating paths with different criteria.
- 6. What happens if a bandwidth value is very high (e.g., > 10 Gbps)?
- In the classic metric formula, the bandwidth component (107 / BW) can become less than 1 and round to 0, making the metric insensitive to differences in very high speeds. This is why EIGRP Wide Metrics were developed.
- 7. Does setting the ‘bandwidth’ command change my link speed?
- No. The `bandwidth` command only updates a parameter used by routing protocols like EIGRP and other systems like QoS. It does not alter the physical transmission rate of the interface.
- 8. How do load and reliability fit into the full formula?
- The full, unabridged formula includes terms for interface load (K2) and reliability (K4, K5). However, since these values can change frequently and cause network instability, their corresponding K-values are 0 by default, effectively removing them from the calculation.