When a router learns multiple routes to a specific network, the route with the lowest administrative distance is installed in the routing table. Sometimes the router must select a route from among many, learned via the same routing process with the same administrative distance. In this case, the router chooses the path with the lowest cost or metric to the destination. Each routing process calculates its cost differently and the costs may need to be manually configured in order to achieve load balancing.

If the router receives and installs multiple paths with the same administrative distance and cost to a destination, load-balancing can occur. There can be up to six equal cost routes (a limit imposed by Cisco IOS on the routing table), but some Interior Gateway Protocols (IGPs) have their own limitation. EIGRP allows up to four equal cost routes.

By default, most IP routing protocols install a maximum of four parallel routes in a routing table. Static routes always install six routes. The exception is BGP, which by default allows only one path to a destination.

The range of maximum paths is one to six paths. To change the maximum number of parallel paths allowed, use the following command in router configuration mode:

Router(config-router)#maximum-paths [number]

IGRP can load balance up to six unequal links. RIP networks must have the same hop count to load balance, whereas IGRP uses bandwidth to determine how to load balance.

Three ways to get to Network X:

• E to B to A with a metric of 30
• E to C to A with a metric of 20
• E to D to A with a metric of 45

Router E chooses the second path above, E-C-A with a metric of 20 as it is a lower cost than 30 and 45.

When routing IP, the Cisco IOS offers two methods of load balancing, per-packet and per-destination load balancing. If process switching is enabled, the router will alternate paths on a per-packet basis. If fast switching is enabled, only one of the alternate routes will be cached for the destination address, so all packets in the packet stream bound for a specific host will take the same path. Packets bound for a different host on the same network may use an alternate route, traffic is load balanced on a per-destination basis.

Lab Activity Lab Exercise: Load Balancing Across Multiple Paths This lab is to configure Load balance across multiple paths.

Interactive Media Activity Drag and Drop: Administrative Distances After completing this activity, the student will be able to administrative distances.

Web Links How Does Load Balancing Work? http://www.cisco.com/en/US/tech/ tk826/tk365/ technologies_tech_note09186a0080094820.shtml