Date: June 28, 2024  |  Author: MD.ANAS

EIGRP Unequal-Cost Load Balancing

EIGRP is an advanced distance vector routing protocol that blends features from both distance vector and link-state protocols. One of its standout capabilities is unequal-cost load balancing, unlike most traditional IGPs which only support equal-cost multi-path (ECMP).

EIGRP allows routers to include routes in the routing table that have different metrics to a destination, thereby improving network efficiency and flexibility.

Variance: The Key to Unequal-Cost Load Balancing

The mechanism that enables unequal-cost load balancing in EIGRP is the variance command, which acts as a multiplier to the best route's metric.

But first, understanding how EIGRP selects the best path is essential.

Composite Metric in EIGRP

EIGRP uses a composite metric that primarily considers:

• Bandwidth (lowest bandwidth along the path) – K1
• Delay (total delay across links) – K3
• Reliability – K4 (optional, rarely used)
• Load – K2 (optional, rarely used)
• MTU – (only used to break ties, not in metric calculation)

Default K Values

Component	Default Value
K1 (Bandwidth)	1
K2 (Load)	0
K3 (Delay)	1
K4 (Reliability)	0
K5 (Reliability)	0

Metric Formula

By default, only bandwidth and delay (K1 & K3) are used to calculate the metric.

Successor and Feasible Successor

EIGRP ensures fast convergence by precomputing backup paths:

• Successor: The best path to the destination, installed in the routing table.
• Feasible Successor: A backup route that satisfies the feasibility condition — its Advertised Distance (AD) from the neighbor must be less than the Feasible Distance (FD) of the current successor.

A route that fails this condition cannot be a feasible successor even if its metric is within the variance range.

Example 1

Path	Feasible Distance (FD)	Advertised Distance (AD)	Metric	Feasible Successor?
Path-1 (Primary)	30	20	100	Successor
Path-2 (Alt)	40	25	150	Yes
Path-3 (Alt)	40	35	200	No

Applying the Variance Command

Router(config)# router eigrp 100
Router(config-router)# variance 3
      

Note:

• Variance = 3
• Best metric = 100 → 100 × 3 = 300
• Any feasible successor with a metric ≤ 300 can be used for load balancing.

So, in this example:

• Path-2 qualifies: AD < FD (25 < 30) and Metric = 150
• Path-3 does not qualify: AD > FD (35 > 30), despite Metric = 200 being in range.