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