Variance is used when we want to apply unequal cost load sharing. Remember that EIGRP can support load sharing up to 6 links. In order for this load sharing to apply, the result of variance value (X) multiplied by the FD of the successor must be bigger than the FD of the feasible successor. Nevertheless, make sure that the variance value is as small as possible, since big variance value can bring some problems. When the variance command is applied, load sharing will be applied, but there is no change in the FD of the successor nor the feasible successor. This can be verified by the command #show ip eigrp topology.
Applying variance is easy using the command:
(config)#router eigrp (eigrp AS number)
(config-router)#variance
On the EIGRP topology table there are 2 values which is the FD (feasible distance) and the AD (advertised distance) or also known as reported distance.
- The FD (first value) is the full metric from the router to the destination network.
- The AD (Second value) is the metric from the next hop of the router to the destination network. Therefore, in most times the AD is lower than the FD.
If the AD has a bigger value than the FD, then there is a possibility of an routing loop. This rule is checked before the successor and feasible successor is applied to the routing table. If this condition is found, the router will not let that route take place into the election of a route or feasible route. Another condition that must be met for the potential feasible successor is that the AD of the feasible successor must be smaller than the FD of the successor. In a condition that where there are no feasible successors, the route on the router will go to the active state and in the meanwhile send DUAL Query Packets.
-- 1 February 2011 --
Applying variance is easy using the command:
(config)#router eigrp (eigrp AS number)
(config-router)#variance
On the EIGRP topology table there are 2 values which is the FD (feasible distance) and the AD (advertised distance) or also known as reported distance.
- The FD (first value) is the full metric from the router to the destination network.
- The AD (Second value) is the metric from the next hop of the router to the destination network. Therefore, in most times the AD is lower than the FD.
If the AD has a bigger value than the FD, then there is a possibility of an routing loop. This rule is checked before the successor and feasible successor is applied to the routing table. If this condition is found, the router will not let that route take place into the election of a route or feasible route. Another condition that must be met for the potential feasible successor is that the AD of the feasible successor must be smaller than the FD of the successor. In a condition that where there are no feasible successors, the route on the router will go to the active state and in the meanwhile send DUAL Query Packets.
-- 1 February 2011 --
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