EIGRP Metric Calculator
Because EIGRP uses the _____ bandwidth in the metric calculation
Total EIGRP Metric (Classic)
Formula: 256 * [(10^7 / Slowest Bandwidth) + (Cumulative Delay / 10)]
Metric Component Weighting
Visualizing how bandwidth vs. delay contributes to the final metric.
What is eigrp uses the _____ bandwidth in the metric calculation?
The phrase “eigrp uses the _____ bandwidth in the metric calculation” is a fundamental concept in Cisco networking, specifically referring to the minimum path bandwidth. Unlike other protocols that might sum bandwidth or use average throughput, Enhanced Interior Gateway Routing Protocol (EIGRP) looks for the single slowest link along the entire path from the source to the destination. This “bottleneck” bandwidth is the value used in the composite metric formula.
Network engineers and CCNA candidates should use this concept to predict how traffic will flow through a complex topology. A common misconception is that EIGRP adds the bandwidth of all links together; however, eigrp uses the _____ bandwidth in the metric calculation (the bottleneck) to ensure that the metric reflects the actual throughput limitations of the path.
EIGRP Metric Formula and Mathematical Explanation
The EIGRP classic metric calculation is a composite formula that involves several variables, though by default, only bandwidth and delay are active. The full logic highlights why eigrp uses the _____ bandwidth in the metric calculation.
The standard formula is: Metric = [K1 * Bandwidth + (K2 * Bandwidth) / (256 - Load) + K3 * Delay] * [K5 / (Reliability + K4)] * 256.
With default K-values (K1=1, K3=1, others=0), it simplifies to:
Metric = 256 * ((10^7 / Minimum Bandwidth in kbps) + (Cumulative Delay / 10))
| Variable | Meaning | Unit | Typical Range |
|---|---|---|---|
| Minimum Bandwidth | Slowest link in the path | kbps | 64 – 100,000,000 |
| Cumulative Delay | Sum of all interface delays | Tens of µs | 10 – 255,000 |
| Reliability | Quality of the link | Fraction (1-255) | 255 (Max) |
| Load | Amount of traffic on link | Fraction (1-255) | 1 (Min) |
| MTU | Maximum Transmission Unit | Bytes | 1500 |
Practical Examples (Real-World Use Cases)
Example 1: T1 Link Path
Suppose a path consists of three routers connected by two links. One link is a Gigabit Ethernet (1,000,000 kbps) and the other is a T1 (1,544 kbps). Since eigrp uses the _____ bandwidth in the metric calculation, we use 1544 kbps as our bandwidth variable. If the cumulative delay is 20,000 microseconds (2,000 tens of microseconds), the calculation is:
- BW Component: 10,000,000 / 1544 = 6476
- Delay Component: 20,000 / 10 = 2000
- Metric: (6476 + 2000) * 256 = 2,169,856
Example 2: Fast Ethernet Path
Imagine a path with four Fast Ethernet links (100,000 kbps each). The minimum bandwidth is 100,000 kbps. Each link has a delay of 100 microseconds, so cumulative delay is 400 microseconds.
- BW Component: 10,000,000 / 100,000 = 100
- Delay Component: 400 / 10 = 40
- Metric: (100 + 40) * 256 = 35,840
How to Use This EIGRP Metric Calculator
- Identify the Bottleneck: Look at your path and find the link with the lowest speed. Since eigrp uses the _____ bandwidth in the metric calculation, this is your primary input.
- Sum the Delays: Add up the delay values of every egress interface along the path to the destination.
- Input the Values: Enter these into the “Minimum Path Bandwidth” and “Cumulative Delay” fields.
- Check K-Values: By default, this tool assumes K1 and K3 are 1. If you have custom reliability or load settings, ensure they are entered correctly.
- Analyze the Result: Use the “Metric Component Weighting” chart to see if your metric is being driven more by slow bandwidth or by long delays.
Key Factors That Affect eigrp uses the _____ bandwidth in the metric calculation Results
- Bandwidth Bottlenecks: Even a single slow link (like a backup 64kbps line) can drastically increase the metric because eigrp uses the _____ bandwidth in the metric calculation.
- Cumulative Delay: Unlike bandwidth, delay is additive. Every hop adds to the total delay metric.
- Interface Scaling: With Gigabit and 10G interfaces, the classic EIGRP metric can “max out” because 10^7 / 10,000,000 equals 1. This is why Wide Metrics were introduced.
- K-Value Constants: If K-values are mismatched between neighbors, the EIGRP adjacency will not form.
- Reliability Impact: While usually ignored, a drop in reliability (below 255) can theoretically trigger a route recalculation if K4/K5 are active.
- Traffic Load: Dynamic load balancing is possible but rarely used because it causes metric flapping and routing instability in most production environments.
Frequently Asked Questions (FAQ)
Why does EIGRP use the minimum bandwidth?
It ensures the metric accurately reflects the maximum possible throughput. Since a path’s capacity is limited by its slowest link, eigrp uses the _____ bandwidth in the metric calculation to avoid overestimating path quality.
Does EIGRP use the same bandwidth for the return path?
Not necessarily. EIGRP metrics are calculated per-direction based on the outbound interface characteristics along the path to the destination prefix.
What happens if two paths have the same minimum bandwidth?
EIGRP will then differentiate between the paths using the Cumulative Delay. If those are also equal, it may perform Equal Cost Multi-Path (ECMP) load balancing.
How does “Wide Metrics” change this?
Wide Metrics (used in EIGRP named mode) use a higher scale (64-bit) and a different constant (10^13) to better accommodate high-speed interfaces like 40G or 100G.
Can I manually change the bandwidth used in the calculation?
Yes, by using the bandwidth command on an interface. This doesn’t change the physical speed but tells EIGRP to use that value for its calculations.
Is MTU used in the EIGRP metric?
MTU is carried in the EIGRP update packets but is NOT actually used in the mathematical calculation of the composite metric.
What is the “256” multiplier for?
The original IGRP protocol used an 8-bit metric. EIGRP scales this by 256 to create a 32-bit metric, allowing for more granular path comparisons.
How does EIGRP handle a bandwidth of 0?
The calculation would technically result in an infinite metric (division by zero), but Cisco IOS prevents setting a bandwidth of 0 on active interfaces.
Related Tools and Internal Resources
- Understanding EIGRP K-Values – A deep dive into how K1 through K5 affect routing.
- Cisco Routing Protocols Compared – How EIGRP, OSPF, and BGP metrics differ.
- Feasible Distance Calculator – Calculate FD and RD for EIGRP successor selection.
- EIGRP Wide Metrics Guide – Learn how 64-bit metrics work for high-speed links.
- Interface Delay Optimization – Best practices for setting interface delay in EIGRP.
- Network Bottleneck Analysis – Identifying why eigrp uses the _____ bandwidth in the metric calculation in your topology.