Advanced Path Tuning & Load Balancing

Deep dive into EIGRP's load-balancing and metric manipulation engine. Unequal-cost load balancing via variance, share-count ratios, offset-list metric inflation rules, inbound vs outbound propagation behavior, and maximum-path tuning for optimal bandwidth utilization.

Author: Hrishi Pradhan

Level: Basic to Advanced

Sections: 4 Theory

Diagrams: 4 Mermaid

Recommended: Complete Module B first

1. Unequal-Cost Load Balancing via Variance Multiplier

How EIGRP uses paths with different metrics simultaneously

Section 1

What, Where, How & Why

Variance allows EIGRP to install multiple paths to the same destination even when their metrics differ, unlike OSPF and RIP which require identical metrics for load balancing. Use it in enterprise networks to utilize all available bandwidth across unequal-cost WAN links. It solves the problem of idle backup links waiting for a primary failure.

%%{init: { 'theme': 'base', 'themeVariables': { 'fontSize': '12px' }}}%% flowchart TD Start["Successor installed
FD = 158720"] --> Multi["Multiply FD by variance
158720 x 2 = 317440"] Multi --> Check{"Is candidate metric
≤ 317440?"} Check -->|Yes| Install["Install candidate
as load-share path"] Check -->|No| Reject["Candidate remains
Alternate only"] Install --> End1["Equal distribution
based on share-count"] Reject --> End1 style Start fill:#7C3AED,color:#FFF style Multi fill:#F59E0B,color:#FFF style Install fill:#10B981,color:#FFF style Reject fill:#EF4444,color:#FFF

Variance Formula: Successor FD × variance ≥ Candidate metric
Variance Range: Default = 1 (equal-cost only), Maximum = 128
Integer Only: Variance cannot be decimal or fractional
Candidate Path: Must first satisfy the Feasibility Condition to be eligible
Result: Both paths installed; traffic distributed proportionally

Worked Example

Path	Metric	RD vs FD	Eligible?	Result
Via R2 (primary)	158720	N/A	Yes	Successor
Via R3 (variance 2)	230400	156160 < 158720	Yes	Installed (load share)
Via R4 (variance 2)	350000	200000 > 158720	No	Fails Feasibility — not eligible

With variance 2 on the router:

R3's metric (230400) is ≤ FD × variance (317440) → install
R4's metric (350000) is > 317440 → do not install
R4 also fails Feasibility (RD > FD) → never eligible regardless of variance

2. Share Count & Proportional Traffic Distribution

How EIGRP decides how much traffic each path receives

Section 2

What, Where, How & Why

Once multiple paths are installed, EIGRP does not split traffic 50/50. It uses a share-count ratio based on path metrics: lower-metric paths carry proportionally more packets. This solves the problem of slow links being overloaded and fast links being underused.

%%{init: { 'theme': 'base', 'themeVariables': { 'fontSize': '12px' }}}%% flowchart LR Traffic["Traffic to
10.1.10.0/24"] --> Distribute{"EIGRP
Share Count
Ratio"} Distribute -->|"4 of 5 packets"| Path1["Path 1 (FD = 100)"] Distribute -->|"1 of 5 packets"| Path2["Path 2 (FD = 400)"] style Traffic fill:#7C3AED,color:#FFF style Distribute fill:#F59E0B,color:#FFF style Path1 fill:#10B981,color:#FFF style Path2 fill:#0EA5E9,color:#FFF

Formula: Share Count = Highest Metric / Path Metric
Example: Path A metric = 100, Path B metric = 400
Path A: 400 / 100 = 4 shares
Path B: 400 / 400 = 1 share
Result: Out of every 5 packets, 4 go via A and 1 via B

Practical Table

Path	Metric	Share Count	Traffic Ratio
Path A (primary)	158720	2	66%
Path B (variance)	317440	1	33%
Total	—	3	100%

Important: Share count is not the same as bandwidth. A 4:1 ratio does not mean a path is 4× faster — it means EIGRP sends 4 packets on one path for every 1 on the other. Actual throughput depends on link speed.

3. Offset-List Metric Inflation & Directional Rules

Per-route metric manipulation for path control and load balancing

Section 3

What, Where, How & Why

Offset-lists increase the metric of specific routes, allowing engineers to selectively make certain paths less preferred. Use them to force traffic through a specific link, enable per-route unequal-cost load balancing, or prevent a route from becoming a Feasible Successor. They solve the problem of tuning metrics without affecting all routes on an interface.

%%{init: { 'theme': 'base', 'themeVariables': { 'fontSize': '12px' }}}%% flowchart TD ACL["ACL matches route
permit 10.1.10.0 0.0.0.255"] --> Offset["offset-list ACL in
+1000 on Gi0/1"] Offset --> InCheck{"Direction?"} InCheck -->|Inbound| Direct["Add offset directly
to local metric"] InCheck -->|Outbound| Downstream["Neighbor sees
offset ÷ 256
rounded up"] Direct --> Topology["Topology table
updated"] Downstream --> Topology Topology --> Result["Successor / FS
recalculated"] style ACL fill:#7C3AED,color:#FFF style Offset fill:#F59E0B,color:#FFF style Direct fill:#10B981,color:#FFF style Downstream fill:#0EA5E9,color:#FFF

Core Rule: Offset-lists can only increase metrics, never decrease
ACL Purpose: Matches the route, NOT filters traffic
Inbound: Adds to the local metric directly (visible immediately)
Outbound: Affects downstream routers; offset is divided by 256 and rounded up
Multiple Uses: Path control, load balancing, preventing FS

Directional Behavior — Critical Difference

Direction	Where Applied	Visibility	Use Case
Inbound (in)	When receiving a route from a neighbor	Changes local metric on this router	Force traffic to prefer a different path on this router only
Outbound (out)	When advertising a route to a neighbor	Downstream routers see the inflated metric	Influence path selection on neighbors without changing local behavior

Configuration Example

! Step 1: Match the specific route with a standard ACL
ip access-list standard MATCH_ROUTE
 permit 10.1.10.0 0.0.0.255

! Step 2: Apply offset in the desired direction
router eigrp 1234
 offset-list MATCH_ROUTE in 1000 GigabitEthernet0/1
 ! This increases the local metric for 10.1.10.0/24 via Gi0/1 by 1000

! Step 3: Verify the change
show ip eigrp topology | include 10.1.10.0

Production Warning

Offset-lists can accidentally destroy a Feasible Successor. If you inflate a backup path's metric so that its RD exceeds the FD, the path will fail the Feasibility Condition and enter Active state during a future failure. Always verify with show ip eigrp topology after applying an offset-list.

4. Maximum-Path Tuning & Equal-Cost Load Balancing

How many paths can EIGRP install, and when does it use them?

Section 4

What, Where, How & Why

The maximum-path command controls how many equal-cost paths EIGRP will install in the routing table. Older IOS defaults to 4 paths; IOS v15 and later support up to 32. This solves the problem of a low default limit hiding available bandwidth on dense mesh topologies.

%%{init: { 'theme': 'base', 'themeVariables': { 'fontSize': '12px' }}}%% flowchart LR Dest["Destination
10.1.10.0/24"] --> P1["Path 1
Metric = 100"] Dest --> P2["Path 2
Metric = 100"] Dest --> P3["Path 3
Metric = 100"] Dest --> P4["Path 4
Metric = 100"] P1 --> RT["Routing Table
Installs all 4
Equal-Cost"] P2 --> RT P3 --> RT P4 --> RT style Dest fill:#7C3AED,color:#FFF style RT fill:#10B981,color:#FFF style P1 fill:#0EA5E9,color:#FFF style P2 fill:#0EA5E9,color:#FFF style P3 fill:#0EA5E9,color:#FFF style P4 fill:#0EA5E9,color:#FFF

Equal-Cost: Paths with identical metrics install automatically
Default max-path: 4 (IOS classic); 16–32 in modern IOS
Traffic Distribution: Round-robin per flow (CEF-based)
No Variance Needed: Only maximum-paths is relevant
Combined with Variance: Variance handles unequal costs; max-path handles quantity

Configuration

router eigrp 1234
 maximum-paths 8
 ! Allows up to 8 equal-cost paths

! To combine with unequal-cost load balancing:
router eigrp 1234
 maximum-paths 8
 variance 2
 ! First, install up to 8 equal-cost paths
 ! Then, install any unequal-cost paths that satisfy the variance rule

Equal-Cost vs Unequal-Cost — When to Use Which

Scenario	Equal-Cost	Unequal-Cost (Variance)
Two links with same bandwidth	Automatic	Not needed
Primary 1 Gbps + Backup 100 Mbps	Not possible (metrics differ)	Use variance
Four equal links + one slower	Use maximum-paths	Combine both commands
FE to backbone with tiered WAN	Unusual — links typically differ	Standard practice

The Golden Rule of EIGRP Load Balancing

Variance handles unequal-cost paths, but the underlying path selection still respects the Feasibility Condition. A path that fails Feasibility cannot be installed regardless of variance. This means proper DUAL topology design (multiple feasible successors) is the foundation for any load-balancing strategy.