EIGRP Metric and K Values Explained

The EIGRP composite metric is the single thing that confuses CCNP candidates the most. It uses a 64-bit value (or 32-bit in legacy mode) calculated from up to five inputs (bandwidth, delay, load, reliability, MTU) weighted by configurable K values. Most engineers stare at the formula, install a route, see a metric like 3,072,000, and never quite understand where the number came from.

This article walks through the metric formula, what each K value controls, why the defaults matter, the wide-metric variant introduced for modern interface speeds, and how to verify metrics in show output. If you are studying for CCNP/CCIE or trying to predict which path EIGRP will pick, this is the math reference.

The Classic Metric Formula

The classic 32-bit EIGRP composite metric (IGRP-derived):

metric = 256 * (K1 * BW + (K2 * BW) / (256 - load) + K3 * delay) * (K5 / (reliability + K4))

Where K5 = 0 means the entire reliability term collapses to 1 (the trailing factor disappears). Default K values: K1=1, K2=0, K3=1, K4=0, K5=0. Substituting:

metric = 256 * (BW + delay)

And BW (bandwidth) here is calculated as 10^7 / minimum-bandwidth-along-path-in-kbps. Delay is sum-of-delays-along-path-in-tens-of-microseconds.

So the simplified default formula:

metric = 256 * ((10^7 / min_bandwidth_kbps) + sum_delay_tens_of_us)

A Worked Example

R1 to a destination over a path with three links:

Minimum bandwidth along the path: 1,544 kbps (the T1).

Sum of delays: 1 + 10 + 2,000 = 2,011 (tens of microseconds).

Calculate:

BW   = 10^7 / 1544 = 6477 (rounded)
metric = 256 * (6477 + 2011) = 256 * 8488 = 2,172,928

That is the EIGRP metric R1 sees for this destination. Verify with show ip eigrp topology.

Where the Bandwidth Comes From

EIGRP uses the interface's configured bandwidth, not actual link speed. The default for most modern Ethernet is set automatically (1,000,000 kbps for GigabitEthernet, etc.) but on serial interfaces it defaults to 1,544 kbps regardless of physical capacity.

Set bandwidth explicitly when needed:

interface Serial0/0/0
 bandwidth 50000          ! 50 Mbps logical bandwidth for EIGRP metric

Important: bandwidth on the interface is purely a routing-protocol hint. It does not affect actual physical throughput, QoS, or packet rate. EIGRP and other protocols use it for metric calculations.

Where the Delay Comes From

Each interface has a default delay value (in microseconds). Cisco's defaults:

EIGRP sums delay along the entire path, expressed in tens of microseconds. Override with:

interface GigabitEthernet0/0/0
 delay 1                  ! 10 us = 1 in tens of us

Tuning delay is the standard way to influence EIGRP path selection without changing bandwidth (which can affect QoS calculations). Increase delay on a path to make it less preferred; decrease delay on a path to make it more preferred.

K Values

The Cisco recommendation: never change K values from defaults. Two reasons. First, all routers in the same EIGRP AS must have identical K values or neighbor relationships fail. Second, including dynamic factors like load and reliability in the metric causes routes to flap as load changes, leading to constant DUAL recomputation.

If you must tune EIGRP path selection, change interface delay or bandwidth instead of K values.

The 64-bit Wide Metric

The classic 32-bit metric saturates at modern interface speeds. A 100 Gbps link has effectively the same metric as a 1 Gbps link because 10^7 / 100,000,000 rounds to 0 in integer math. To address this, IOS 15.x introduced the 64-bit "wide metric":

metric = (K1 * latency + (K2 * latency)/(256-load) + K3 * BW) * (K5/(K4+reliability)) * 65536

The wide metric uses different units (latency in picoseconds, throughput in 65536 * kbps) and a 64-bit final value, preventing saturation at 100 Gbps and beyond. Enabled by default in modern IOS XE; older networks may run in classic mode for backwards compatibility.

Verify metric mode with:

Router# show ip eigrp topology
EIGRP-IPv4 Topology Table for AS(100)/ID(1.1.1.1)
        Internal     -  Cost: 25 / 10                      <-- Classic
        Internal     -  Cost: (1024000 [12345 us])         <-- Wide

The wide metric output shows latency in microseconds explicitly, making it easier to interpret.

no auto-summary: Why It's Mandatory

Default behavior on legacy IOS: EIGRP automatically summarizes routes at classful boundaries. A router with networks 192.168.1.0/24, 192.168.2.0/24, and 192.168.3.0/24 advertises all three as 192.168.0.0/16 across an inter-classful link.

This is broken for modern VLSM-using networks. Different /24s belong to different sites; auto-summary mashes them together and breaks routing. Always disable:

router eigrp 100
 no auto-summary

Modern IOS XE has no auto-summary enabled by default in named-mode configurations. Classic-mode configs may still inherit the old default; always check.

Manual Route Summarization

Where auto-summary is dangerous, manual summarization at administrative boundaries is essential. EIGRP supports per-interface summarization:

interface GigabitEthernet0/0/0
 ip summary-address eigrp 100 10.0.0.0 255.255.0.0

Summary's metric defaults to the lowest metric among the summarized routes. Useful for stub-area-style behavior and reducing the size of the routing table at boundaries.

For named-mode EIGRP:

router eigrp PROD
 address-family ipv4 unicast autonomous-system 100
  af-interface GigabitEthernet0/0/0
   summary-address 10.0.0.0 255.255.0.0

Verifying Metrics

Router# show ip eigrp topology 10.10.10.0/24
EIGRP-IPv4 Topology Entry for AS(100)/ID(1.1.1.1) for 10.10.10.0/24
  State is Passive, Query origin flag is 1, 1 Successor(s), FD is 2172928
  Routing Descriptor Blocks:
  10.0.13.3 (GigabitEthernet0/0/1), from 10.0.13.3, Send flag is 0x0
      Composite metric is (2172928/10), route is Internal
      Vector metric:
        Minimum bandwidth is 1544 Kbit
        Total delay is 20110 microseconds
        Reliability is 255/255
        Load is 1/255
        Minimum MTU is 1500
        Hop count is 3
        Originating router is 3.3.3.3

The Vector metric shows you the inputs to the calculation. Minimum bandwidth (1544 kbps along the path), Total delay (20,110 us), and so on. The Composite metric (2172928) is the result. Verify your math against this output.

Anti-Patterns

• Changing K values to "tune" path selection. Never. All routers must agree, and dynamic K values cause flapping.
• Changing bandwidth to influence routing. Better than K-value tuning but watch for QoS implications - the bandwidth statement also affects QoS calculations on the same interface.
• Leaving auto-summary enabled. Causes classful-boundary mash-ups in any VLSM-using network. Always no auto-summary.
• Forgetting to disable interface bandwidth on cables that share T1 with other services. The default 1,544 kbps drives EIGRP's choice; if you have a 100 Mbps logical channel inside a fractional T1, set bandwidth explicitly.
• Mixing classic and wide metric routers in the same AS. Modern IOS XE handles the conversion but edge cases exist. Standardize.

Summary

EIGRP's composite metric is bandwidth + delay (with default K values), where bandwidth is 10^7 / minimum-bandwidth-along-path and delay is sum of interface delays. The math is unintuitive at first but predictable once you walk through it. The wide metric extends this to 64 bits for modern interface speeds without changing the conceptual formula.

Never change K values. Tune via interface delay or bandwidth. Always no auto-summary. Use manual summarization at boundaries. Bookmark this article alongside the EIGRP cluster pillar and the DUAL deep-dive.