Load Balancing

Load Balancing
Load balancing is a way that a router can send traffic over multiple paths to the same destination.
It is used to cut down on the amount of traffic passing over a single path to a destination. IGRP, by
default, is set to load-balance across four equal-cost paths, meaning four paths with equal metrics.
IGRP can be configured to support a single path or simultaneous use of two to six equal- or unequalcost
paths. In order to change the number of paths for load-balancing IGRP, the maximum-paths
number_of_paths command must be entered in router configuration mode:
Router(config-router)#maximum-paths ?
<1-6> Number of paths
Load balancing for IGRP and EIGRP differs from other routing protocols. IGRP and EIGRP
can load-balance across unequal-cost paths, where other routing protocols require load balancing
across equal-cost paths. This means that multiple paths that do not have the same metric can be
used for load balancing. Unequal-cost load balancing is made possible through the concept of
route variance. The variance is a multiplier that is used to determine what the acceptable metric
for a route is for it to be included in the routing table. In order to configure the variance, enter the
variance multiplier command in router configuration mode:
Router(config-router)#variance ?
<1-128> Metric variance multiplier
The path with the lowest metric is entered into the routing table, as always. The variance is
then applied to the lowest metric to determine what other routes can be included in the routing
table. If the variance is set to 1, as it is by default, then only equal-cost routes will be accepted.
There are actually two requirements that must be satisfied, in order for alternate routes to be
admitted into the routing table:
 The first requirement is the obvious one. The metric for the route through the neighboring
router must be less than or equal to the product of the variance times the lowest local metric
for the same destination network.
 The second requirement is not known by most, but is no less imperative. Unlike RIP which adds
a hop to its own metric in an advertisement for a route, an IGRP or EIGRP router advertises its
own metric to its neighbor, leaving the calculation of the additional metric value to the receiving
neighbor. This behavior is due to the fact that only the receiving neighbor knows of the outbound
variables that contribute to the final composite metric, from its point of view, for any
given route. The advertised metric is actually the metric used by the advertising router in its
routing table. That brings us to the second requirement. This advertised metric must be less than
the lowest metric that the local router currently uses for the same destination network.
All this means it’s possible that, even though an alternate route’s metric is less than or equal
to the product of the variance and the best local route, a route would not be used for unequalcost
load balancing if the advertising neighbor is farther away from the destination network
than the local router is. This is a safeguard to avoid routing loops. If the neighboring router is
farther from the destination than the local router is, perhaps the local router is the next hop for
the neighbor. We definitely wouldn’t want to perpetuate such a loop by sending a portion of our
traffic through such a path.
110 Chapter 4  IGRP and EIGRP
The other ramification of all of this is that if a neighbor is used by the local router as the next
hop of an alternate route to a destination network, then conversely the local router will not be
selected by that neighbor as the next hop for the same destination network. This is because the
rule does not allow for the advertised metric to be equal to the local metric. It must be better,
meaning the local metric is worse, which would not satisfy this requirement for the neighbor.
Once the paths have been selected, the traffic is then divided up according to the actual metric
of each path. For example, let’s imagine that the path with the highest metric for load balancing
is four times greater than the path with the lowest metric. For every one packet that is sent across
the higher metric path, four packets will have been sent across the lower metric path.