6 Replies Latest reply: Oct 15, 2015 7:11 PM by Dmcneil RSS

    EIGRP Variance Command

    mohammed

      Hi all

      I have a doubt what does the value of variance commnad really mean ?

       

      if it was 1, 2 or 3 what these numbers mean ?

       

      thanks,

        • 1. Re: EIGRP Variance Command
          Rick

          There is a great explanation of that here:

           

          https://learningnetwork.cisco.com/thread/43785

          • 2. Re: EIGRP Variance Command
            Elvin Arias

            For more information about the variance command visit. http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a008009437d.shtml

             

            Elvin

            • 3. Re: EIGRP Variance Command
              Gurudath KS

              Eigrp is the only protcol which supports unequal load balancing..

               

              If Router A can reach Router B in three different links,with a cost metric of 10,20,30 then router A selects path with cost metric 10  to reach Router B,

               

              Now if you Put varience Command with 2, then

              Lowest cost metric 10 *2=20

               

              Now, Router shares traffic with metric 10 & 20 link.

              • 4. Re: EIGRP Variance Command
                apapierz

                You're right but remember that first you have to consider the feasible condition which states that advertised distance (AD) of the second route must be lower than feasible distance (FD) of the current successor route. This helps to prevent routing loops and is taken into account as first.

                 

                In your example you have the best route with FD=10. Configuring variance value of 2 you will be able to use other paths with metric beter or equal to 20 but only if the AD of those paths will be less than 10.

                 

                Regards, Andrzej

                • 5. Re: EIGRP Variance Command
                  daniel

                  So if all your cost are equal the variance command does nothing?

                   

                  The only metric that your variance command changes is the lowest cost metric?

                   

                  what if you have 3 feasible successors all with different cost, does the variance command only apply to the lowest one?

                   

                  How do you choose what number to use?

                   

                  Is the goal of the multiplier to make the lowest cost higher than the other feasible successors?

                   

                  Lastly if I cut a tree down in the forest and I am running OSPF, will EIGRP hear it fall?

                  • 6. Re: EIGRP Variance Command
                    Dmcneil

                    You should look through Elvin's Link it has great information.

                     

                    Variance does not change the Feasible Distance of a route. Variance is most easily understood as "I will accept a route with a Feasible Distance that is at least X times worse than my best route"

                     

                    So using the above examples (assuming the feasibility condition is met with all these routes) If you have routes with Feasible Distances of 10,20,30,40. The route with Feasible Distance of "10" is your best route.

                     

                    Now, with a variance of 2 you are saying. "Any route that is up to 2x's worse than '10' can be considered equal". So This means the route with Feasible Distance of 10 and 20 would be allowed. However, 30 and 40, would not become successors because they are more than 2x's worse than your best route of 10.

                     

                    If you change the variance to 3. Then you are saying "Any route that is up to 3x's worse than '10' can be considered equal". So 10,20, and 30 will be accepted as successor routes and sent to the routing table.