Load sharing will occur if the costs are equal

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If the OSPF routing protocol is used, the load backup feature load-shares between the primary and backup links after the backup link is activated. However, the cost assigned to the primary link and the backup link must be equal if both links are used. If one link has a lower cost than the other, all routing will occur over the link with the lower cost, even though both lines are up.

OSPF does not support load balancing between the primary link and the backup connection if the links are not equal. If load balancing is to occur in this environment, the backup connection must be able to support comparable bandwidth environments. (For example, a 64-kbps ISDN connection backs up a 64-kbps serial connection.)

Load Sharing with EIGRP

© 2004 Cisco Systems, In

If EIGRP is used, the load backup feature will load-share between the primary and backup links after the backup link is activated. However, the metric assigned to the primary link and the backup link must be equal if both links are to be used. If one link has a lower metric than the other, all routing will occur over the link with the lower metric even though both lines are up. If load balancing is to occur in this environment, each connection must be able to support comparable bandwidth environments. (For example, a 64-kbps ISDN link backs up a 64-kbps serial connection.)

Instead of relying on equal metrics to load-share and load-balance, the variance configuration command can also be used to control load balancing in an 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. An oversimplified explanation is that a router can use paths with worse routing metrics up to a value less than the current best route metric times the variance.

variance multiplier Command

Command

Description

multiplier

The range of metric values that will be accepted for load balancing. Acceptable values are nonzero, positive integers. The default value is 1, which means equal-cost load balancing. In the example, the multiplier is set to 2.

Setting this value lets the router determine the feasibility of a potential route.

Setting this value lets the router determine the feasibility of a potential route.

If the following two conditions are met, the route is deemed feasible and can be added to the routing table for load sharing:

Local best metric (current FD) > best metric (AD) learned from the next router. This condition exists if the next router in the path is closer to the destination than the current router. This approach prevents routing loops.

■ The variance number multiplied by the local best metric (current FD) > metric (FD) through the next router. This condition is true if the metric of the alternate path is within the variance.

In the figure, the variance 2 command specifies to use both paths even if the metric of the backup path is two times worse than the primary path.

You can use the traffic-share {balanced | min} command to control how traffic is distributed among EIGRP load-sharing routes. The default is four routes and the maximum is six routes.

The traffic-share balanced command distributes traffic proportionally to the ratios of the metrics. As a result of the variance 2 command, the best route will transport two times the traffic of the worst route. The traffic-share min command specifies to use routes with the least cost.

Note Advertised distance (AD) is the metric that a neighbor uses to reach a given destination network. The AD is advertised as part of the EIGRP update for a given network. A router receiving the update adds its cost to reach that neighbor to the AD. The sum of these values provides the feasible distance (FD) to reach that destination network through that neighbor router.

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