Back to EIGRP GuideENARSI path control

EIGRP Load Balancing: Equal Cost, Variance and Verification

Learn why EIGRP installs more than one path, when an unequal path is safe, and how to prove which links actually carry traffic.

Equal CostVarianceFeasibilityMaximum PathsTraffic Share

Load Balancing at a Glance

EIGRP can install several paths to the same destination. Equal-cost paths have the same feasible distance. Unequal-cost paths can also be installed, but only when they are loop-free and fall inside the configured variance range.

Default behaviorEqual cost

Unequal-cost controlvariance

Loop-free requirementFeasibility condition

Path limitmaximum-paths

Safe rule: Variance never makes an unsafe path eligible. The path must first satisfy the feasibility condition.

1. Build a Two-Path Lab

R1 can reach the destination behind R4 through R2 or R3. Use different delay values so the two paths have different metrics.

R1Decision routerChooses paths to 10.40.40.0/24

Path Avia R2Metric 20,000

R410.40.40.0/24Destination edge

Path Bvia R3Metric 30,000

Two choicesAS 100Compare FD and RD

Conceptual path comparison. Confirm real values in the topology table before changing variance.

2. Understand Equal-Cost Multipath

When two successor paths have the same best metric, EIGRP can install both. The platform and the configured maximum decide how many paths can be used.

Configure the limit

router eigrp 100
 maximum-paths 4

Verify installed next hops

show ip route 10.40.40.0
show ip cef 10.40.40.0 detail

Do not confuse: Multiple routes in the routing table prove installation. CEF output is better evidence for forwarding behavior.

3. Check the Feasibility Condition First

An alternate neighbor is loop-free when its reported distance is lower than the current successor feasible distance.

Alternate reported distance<Successor feasible distance

Path	FD	Neighbor RD	Result
R1 → R2	20,000	15,000	Successor
R1 → R3	30,000	18,000	Feasible successor
Unsafe example	25,000	22,000	Fails when successor FD is 20,000

4. Calculate and Configure Variance

Variance multiplies the best metric to create an installation ceiling. Choose the smallest whole number that includes the intended feasible successor.

Alternate FD≤Best FD × variance

For a best metric of 20,000 and an alternate metric of 30,000, variance 2 creates a ceiling of 40,000.

router eigrp 100
 variance 2
 maximum-paths 2

5. Understand Traffic Sharing

With balanced sharing, EIGRP gives a better path a larger traffic share. Exact forwarding depends on platform behavior and flow hashing.

Use installed paths

router eigrp 100
 traffic-share balanced

Use minimum-cost paths only

router eigrp 100
 traffic-share min across-interfaces

Operational caution: A feasible path may still be too slow for production. Check capacity, delay, loss, and policy first.

6. Verify from Topology to Forwarding

Topology

Find successors, feasible successors, FD, and RD.

show ip eigrp topology all-links

Routing

Confirm expected next hops are installed.

show ip route prefix

Forwarding

Confirm CEF uses the intended adjacency set.

show ip cef prefix detail

7. Troubleshoot a Missing Alternate Path

Observation	Likely reason	First check
Absent from topology	Not learned or filtered	`show ip eigrp topology all-links`
Present but not feasible	RD fails feasibility	Compare RD with successor FD
Feasible but not installed	Variance or maximum-paths	`show ip protocols`
Traffic looks uneven	Flow hashing or metric ratio	Inspect CEF and counters

8. Guided Practice Lab

Create equal paths

Use matching bandwidth and delay.
Confirm two successor routes.
Record routing and CEF output.

Create an unequal path

Increase delay on one path.
Confirm it remains feasible.
Calculate the smallest variance.

Prove the path limit

Set maximum-paths 1.
Confirm one path remains.
Restore and verify the baseline.

9. Frequently Asked Questions

Does variance create a feasible successor?

No. DUAL decides whether the alternate is loop-free. Variance only controls installation.

Is a larger variance always better?

No. Use the smallest value that includes the intended path.

Why is traffic not split exactly in half?

Forwarding normally hashes flows rather than alternating every packet.