Load Balancing

As we have learned in our CIT1122 class, load balancing is used when a router has multiple paths to a destination when forwarding packets. It is a standard function of the Cisco IOS software for all of the routing protocols we study.

Load balancing occurs when a router receives multiple paths with the same administrative distance and the same cost to a destination. You can set load-balancing to work per-destination or per-packet.

Per-destination load balancing means the router distributes the packets based on the destination address. Given two paths to the same network, all packets for a specific destination go over the first path, all packets for a second destination on that same network would go over the seconed path.

Per-packet load-balancing means that the router sends one packet for destination1 over the first path, the second pacet for the same destination1 over the second path, and so on. Per-packet load balancing guarantees equal load across all links. For per-packet load balancing, the forwarding process determines the outgoing interface for each packet by looking up the route table and picking the least used interface. This ensures equal utilization of the links, but is a processor intensive task.

For most Cisco routers, per-destination load-balancing is the default. You can enable per-packet load-balancing with the ip route-cache command for a specific interface.

If you issue the show ip route command, you will see an asterick corresponding to the active route that is used for new traffic (single packet or entire flow to a destination - depending on what method you are using). For per-packet load-balancing, the asterisk points to the interface over which the nexst packet is sent. For per-destination load-balancing, the asterisk points to the interface over which the next destination flow is sent. Check this out, in the lab, such as Lab 7.5.1. Let me know what you observe - I will be curious as to your findings/reflections.

There is more to know about load-balancing, but this should get you started. You will learn more in your CCNP class about per-packet load-balancing (process-switching) and per-destination load-balancing (fast-switching).

Hope this helps!