EIGRP Load Balancing

Question 1

Load Balancing

Answer 1

EIGRP automatically load balances over equal metric paths.

EIGRP allows multiple successor routes (using the same metric) to be installed into the RIB.

Installing multiple paths into the RIB for the same prefix is called equal-cost multipathing (ECMP).

Question 2

ECMP

Answer 2

Installing multiples paths into the RIB for the same prefix is called equal-cost multipath (ECMP) routing.

Default maximum ECMP paths for IOS are 4 routes and 16 routes for IOS XR.

ECMP can be changed with the command maximum-paths maximum-paths under the EIGRP process to increase the default value to 16 for IOS and 32 for IOS XR.

Question 3

Unequal-cost Load Balancing

Answer 3

EIGRP supports unequal-cost load balancing, which allows installation of both successor routes and feasible successors into the RIB.

EIGRP supports unequal-cost load balancing by changing EIGRP’s variance multiplier.

Question 4

Variance

Answer 4

The EIGRP variance value is the feasible distance (FD) for a route multiplied by the EIGRP variance multiplier.

Any feasible successor’s FD with a metric below the EIGRP variance value is installed into the RIB.

EIGRP installs multiple routes where the FD for the routes is less than the EIGRP multiplier value up to the maximum number of ECMP routes.

Question 5

Variance Formula

How to get the variance multiplier?

Answer 5

Dividing the feasible successor metric by the successor route metric provides the variance multiplier.

The variance multiplier is a whole number, so any remainders should always round up.