| If the routing protocol
used is Interior Gateway Routing Protocol (IGRP) or Enhanced IGRP (EIGRP),
or if you have a static route configured, the load backup feature
will load share between the primary and backup links after the
backup link is activated only if certain conditions are met. The
metric assigned to the primary link and the backup link must be the
same if both links are to be utilized, however. If one link has a
lower metric than the other, all routing occurs over the link with
the lower metric, even if both lines are up. If load balancing is to
occur in this environment, each connection must be able to support
comparable bandwidth environments (a 56 kbps serial backs up a 56
kbps serial connection).
Instead of relying on equal metrics
to load share and load balance, the variance router configuration
command can also be used to control load balancing in an IGRP/EIGRP
environment. Use the variance
multiplier command to
configure unequal-cost load balancing by defining the difference
between the best metric and the worst acceptable metric.
Figure
shows a sample network and the configuration to create load
balancing between primary and backup lines. Figure
covers the variance
command.
Setting this value lets the
communication server determine the feasibility of a potential route.
A route is feasible if the next communication server in the path is
closer to the destination than the current communication server, and
if the metric for the entire path is within the variance. Only paths
that are feasible can be used for load balancing and included in the
routing table.
If the following two conditions are
met, the route is deemed feasible and can be added to the routing
table:
- The local best metric must be
greater than the metric learned from the next communication
server.
- The multiplier times the local
best metric for the destination must be greater than or equal to
the metric through the next communication server.
In Figure ,
Router E will choose the second path, E-C-A with a metric of 20.
This is because a metric of 20 is better than 30 and 45. To instruct
EIGRP to select the path E-B-A as well, configure variance with a
multiplier of 2:
router eigrp 1
network x.x.x.x
variance 2
This increases the minimum allowable
metric to 40 (2 * 20 = 40). EIGRP includes all the routes that have
a metric under 40, and are feasible successors. In the above
configuration, EIGRP will now use two paths to get to Network X,
E-C-A and E-B-A, because both paths have a metric under 40. EIGRP
will not use path E-D-A because it has a metric of 45, and it is not
a feasible successor.
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