Load balancing (equal and unequal cost)

1.9 Troubleshoot EIGRP (classic and named mode; VRF and global)

📘CCNP Enterprise – ENARSI (300-410)

EIGRP (Enhanced Interior Gateway Routing Protocol) is a dynamic routing protocol that can make decisions about which paths to use based on metrics like bandwidth, delay, reliability, and load. One of its strong features is load balancing — the ability to send traffic over multiple paths to the same destination.

1. Equal-Cost Load Balancing

Definition:
Equal-cost load balancing happens when multiple routes to the same destination have exactly the same EIGRP metric. EIGRP can use all of these paths simultaneously to forward traffic, effectively distributing the load.

Key Points:

By default, EIGRP supports up to 4 equal-cost paths.
- You can change this maximum using the maximum-paths command.
EIGRP calculates the metric using the formula: Metric = [K1 * bandwidth + K3 * delay] * 256 (Ignoring K2, K4, K5 for simplicity unless advanced tuning is done.)
Only routes with the same metric are considered equal-cost and eligible for this type of load balancing.

Configuration Example:

router eigrp 100
 network 10.0.0.0
 maximum-paths 6

This increases the number of equal-cost paths EIGRP can use to 6 (default is 4).

Exam Tip: Remember, all paths must have identical metrics to qualify for equal-cost load balancing.

2. Unequal-Cost Load Balancing

Definition:
EIGRP can also perform load balancing over routes that do not have the exact same metric. This is called unequal-cost load balancing. It allows traffic to be sent over less preferred paths but still make use of them efficiently.

How It Works:

Uses a feature called Variance.
The variance is a multiplier that determines which less optimal routes are acceptable.
Formula: Acceptable Metric ≤ (Best Metric × Variance)
- Best Metric = metric of the primary route (successor)
- Variance = multiplier you configure (must be ≥1)
- Routes with metrics less than or equal to this value become feasible successors and are included in load balancing.

Important Notes:

Only routes that satisfy the feasibility condition (FC < FD) are eligible.
Unequal-cost load balancing is disabled by default; you must configure variance manually.

Configuration Example:

router eigrp 100
 variance 2

If the best metric is 1000, EIGRP will use paths with metrics up to 2000 (1000 × 2).
This allows traffic to take backup paths without causing loops.

3. Key Commands for Load Balancing

Command	Purpose
`show ip route eigrp`	See which paths are used and their metrics
`show ip eigrp topology`	View feasible successors and successors
`maximum-paths <number>`	Set number of equal-cost paths to use
`variance <number>`	Enable and set factor for unequal-cost load balancing

4. How Load Balancing Works in IT Networks

In IT environments:

Equal-cost paths: Multiple links between two routers can all be used at the same time to share the traffic load. For example, two 10 Gbps links connecting two data centers may each carry half the traffic.
Unequal-cost paths: If one path is 10 Gbps and another is 5 Gbps, you can still use both by setting a variance of 2. EIGRP will send more traffic over the faster 10 Gbps link and some traffic over the slower 5 Gbps link.

This is crucial for redundancy and efficient bandwidth usage in enterprise networks.

5. Exam Tips for CCNP ENARSI

Know the difference between equal-cost and unequal-cost load balancing.
Remember maximum-paths controls the number of equal-cost paths.
Remember variance enables unequal-cost paths and how it works mathematically.
Only routes meeting the feasibility condition can be used in unequal-cost load balancing.
Commands to check: show ip route, show ip eigrp topology, show running-config.

✅ Summary Table:

Feature	Equal-Cost	Unequal-Cost
Metric requirement	Must be identical	Can differ, within variance
Default behavior	Enabled	Disabled
Key command	`maximum-paths`	`variance`
Maximum paths	Default 4	N/A (based on variance)
Use case	Load sharing over identical links	Load sharing and redundancy over different-speed links

This covers everything you need for the CCNP ENARSI exam on EIGRP load balancing.