| EIGRP LOAD BALANCING | | | | all the routes is unlikely due to the fact that |
| Load balancing also referred to as load sharing | | | | EIGRP uses such an unwieldy metric. |
| gives the network the ability to use the | | | | With the variance command you can instruct |
| bandwidth which is available on links which would | | | | EIGRP to consider Feasible Successors as viable |
| have been only used in the event of the primary | | | | routes to be placed into the routing table along |
| link failing. | | | | side the Successor route. |
| Another use of Load balancing is much faster | | | | The variance is a multiplier which allows you to tell |
| convergence since there are multiple routes in the | | | | the EIGRP process to consider Feasible |
| routing table with the same prefix, in actual fact | | | | Successors which have Feasible Distances that fall |
| the convergence time in testing has been shown | | | | within the value of the multiplier. |
| to be essentially instant. | | | | The way the variance works is you set the |
| As an example imagine we have a branch router | | | | multiplier as a number between 1 and 128. The |
| with two routes to towards the headquarters | | | | EIGRP process will then multiply the Successor's |
| prefix, if one of the routes was to fail EIGRP | | | | route Feasible Distance by the value of the |
| would not need to look for a Feasible successor in | | | | variance multiplier and any Feasible Successors |
| the topology nor will it go active on the route | | | | whose Feasible Distance is less than the result of |
| since the other route is already in the routing | | | | the variance multiplied by the Feasible distance will |
| table. (For the record EIGRP will only look in the | | | | be placed into the routing table and treated as |
| topology table or go active on a route if a prefix | | | | equal to the original route. The number of routes |
| to the network does not exist in the routing | | | | that can be placed into the routing table for the |
| table) | | | | same prefix length is the value set by the |
| Enabling load balancing in EIGRP is quite simple and | | | | "maximum-path" command which as you'll recall is |
| requires only two commands, the first of these | | | | 4. |
| commands is configured under the EIGRP path: | | | | One point to always bear in mind is that unless a |
| Router(conf-router)#maximum-path number | | | | route in the topology table the path cannot be |
| This command is defaulted to 4 paths which is | | | | used to load balance across. For the route to |
| generally seen as big enough since most networks | | | | make it into the topology table the Advertised |
| would not have more than 4 possible paths. | | | | distance of the route must be less than the |
| The second command to configuring EIGRP load | | | | Feasible Distance of the Successor route. |
| balancing is the variance command. The variance | | | | Traffic is load balanced across the path |
| command gives you control of an operation | | | | proportionally according to the route metrics with |
| known as a "multiplier". This multiplier allows you to | | | | more traffic being sent across the lower metric |
| get around the problem of EIGRP's rather large | | | | paths, or the router will send all the traffic across |
| metric. Consider a network with 2 or more paths | | | | the best metric path and leaving the other path in |
| through the network to a common destination, | | | | active standby in the event the primary path fails. |
| the chances of EIGRP's metric being identical for | | | | |