EIGRP Unequal load balancing. Less or equal than variance, or just less?

[Temciuc Ion]

I read some documentation and articles about EIGRP variance command:

https://www.cisco.com/c/en/us/support/docs/ip/enhanced-interior-gateway-routing-protocol-eigrp/13677-19.html

https://community.cisco.com/t5/networking-documents/troubleshooting-eigrp-variance-command/ta-p/3129662

From first article: 

                   "Use the variance n command in order to instruct the router to include routes with a metric of less than n times                          the minimum metric route for that destination."

after, in firs example with variance 2:

                  "EIGRP includes all routes that have a metric of less than or equal to 40 and satisfy the feasibility condition. In the                      configuration in this section, EIGRP now uses two paths to get to Network X, E-C-A and E-B-A, because both                          paths have a metric of under 40."

after, in second example with variance 2:

                "Now, assume that the metric between E-B is 25 and the metric between B-A is 15. In this case, the E-B-A metric                   is 40. However, this path will not be selected for load balancing because the cost of this path, 40, is not less than                           (20 * 2 ), where 20 is the FD and 2 is the variance."

Ok, in second example metric is not less than 40 (20*2), but it is equal to 40, why this path will not be selected for load balancing?

[Jaderson Pessoa]

@Temciuc Ion Hello,

The link that you post here is very clear about:

This configuration increases the minimum metric to 40 (2 * 20 = 40). EIGRP includes all routes that have a metric of less than or equal to 40 and satisfy the feasibility condition. In the configuration in this section, EIGRP now uses two paths to get to Network X, E-C-A and E-B-A, because both paths have a metric of under 40. EIGRP does not use path E-D-A because that path has a metric of 45, which is not less than the value of the minimum metric of 40, because of the variance configuration. Also, the reported distance of neighbor D is 25, which is greater than the feasible distance (FD) of 20 through C. This means that, even if variance is set to 3, the E-D-A path is not selected for load balancing because Router D is not a feasible successor.

[Temciuc Ion]

@Jaderson Pessoa  Thanks for answer , but my question is about second example:
"Now, assume that the metric between E-B is 25 and the metric between B-A is 15. In this case, the E-B-A metric is 40. However, this path will not be selected for load balancing because the cost of this path, 40, is not less than (20 * 2 ), where 20 is the FD and 2 is the variance. In order to include this path also in load sharing, the variance should be changed to 3. In this case, the traffic share count ratio is:

For path E-C-A: 40/20 = 2

For path E-B-A: 40/40 = 1"

[simmohamedb]

@Temciuc Ion Hi,

I believe @Jaderson Pessoa has already answer your question. Let me explain using my way...

In EIGRP Variance command is used to load share the traffic via unequal cost path, though there is condition should met for choosing the secondary path when we using variable command. It should be elected as a feasible successor path. 

A feasible successor is a backup path to reach that same destination that can be used immediately if the successor route fails. ... For a route to be chosen as a feasible successor, one condition must be met: the neighbor's advertised distance (AD) for the route must be less than the successor's feasible distance (FD)

In our scenario, considering the path E-C-A is the successor to reach Net X and the total distance is 10 but the advertised distance of path E-D-A is 25 which is higher than the current successor. So its failed the condition to be selected as a feasible successor. 

Come to your question; when we configure the variance as 3 (20*3=60) but it won't consider the E-D-A as a secondary path though the total distance is less than the current FD (60)

Hope this get clarify your question. 

Regards

Mohamed B

[Temciuc Ion]

@simmohamedbHi,
My question is about modified scenario E-25-B-15-A, not E-D-A and its FD,
In link that I post is a modified scenario:
"Now, assume that the metric between E-B is 25 and the metric between B-A is 15. In this case, the E-B-A metric is 40. However, this path will not be selected for load balancing because the cost of this path, 40, is not less than (20 * 2 ), where 20 is the FD and 2 is the variance. In order to include this path also in load sharing, the variance should be changed to 3. In this case, the traffic share count ratio is:

For path E-C-A: 40/20 = 2

For path E-B-A: 40/40 = 1"

Now RD (reported distance) from B is 15, 15<20(FD of Router E), FC (Feasible Condition) is meet and Router B is FS (Feasible Succesor).

Variance is still 2, 

The E-B-A metric is 40, 40 is equal to E-C-A metric, why this path will not be selected for load balancing?

[simmohamedb]

It should be less than the new FD (40) value. That is reason the E-B-A is not choosing as a unequal path.

Regards

Mohamed B