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Advanced DDR Operations 163
In the example, note the backup load 90 5 command, which specifies that the router should
monitor the load on the primary interface and bring the link up when the load across the primary
link is particularly heavy. The numbers represent the load of the interface as shown by the show
interface s0
command.
The load on an interface is represented by a number between 1 and 255. In the backup load 90 5
command, 90 is the percentage load at which the backup link is activated (in this case 230/255).
The second number (in this case, 5%) is a measurement of aggregate load. Once the backup link
has been initialized, the router continues to monitor the load. Once the load of both interfaces
combined reaches a value of 13/255, the secondary link is terminated.
So although dial backup was designed for link redundancy to partially compensate for failure,
it can also provide load sharing capabilities to alleviate congestion on the WAN link.
Alternative Backup
Dial backup is not the only method of providing redundancy. In some implementations, ISDN
is not available or viable. In such cases, alternative methods may be the only option. If multiple
links are available, standard routing protocol operations automatically load balance across
equal cost redundant links.
This load balancing usually requires no configuration. From time to time, such as with IPX RIP,
it is necessary to define the number of alternative pathways that should be allowed from a
specific source to a specific destination. There are even routing protocols that go one step
further.
Dynamic Backup
IGRP and EIGRP have a configuration option known as a variance. These two Cisco proprietary
routing protocols can load balance over a maximum of six redundant pathways. The difference
between the protocols is that the redundant pathways do not have to equal the cost pathways.
The only real rule in the selection is that the next hop must be closer to the destination (that is,
it cannot go back to go forward).
The variance factor determines the amount of traffic to send across these suboptimal routes.
For example, a variance of 4 (e.g., issuing the variance 4 command at the routing protocol
configuration mode) tells the router to send data over a particular route if the metric is within
four times the value of the best route.
Variance is calculated based on the documentation of your network. It should be evident from
the examination of network topological maps exactly how many pathways are available from a
particular source to a particular destination.