Cisco Cisco IOS Software Release 12.0(21)S

The Per-Packet Load Balancing feature allows data traffic to be evenly distributed in an IP network over 
multiple equal-cost connections. Per-packet load balancing uses round-robin techniques to select the 
output path without basing the choice on the packet content.

Load balancing is based on a combination of source and destination packet information; it allows you to 
optimize resources by distributing traffic over multiple paths for transferring data to a destination. You 
configure load balancing on outbound interfaces on a per-destination or per-packet basis. 

Per-destination load balancing allows the router to distribute packets based on the destination address, 
and uses multiple paths to achieve load sharing. Packets for a given source-destination host pair are 
guaranteed to take the same path, even if multiple paths are available. For example, given two paths to 
the same network, all packets for destination1 on that network go over the first path, all packets for 
destination2 on that network go over the second path, and so on. Per-destination load balancing is 
enabled by default when you start the router, and is the preferred load balancing for most situations.

Per-packet load balancing allows the router to send successive data packets over paths without regard to 
individual hosts or user sessions. It uses the round-robin method to determine which path each packet 
takes to the destination. With per-packet load balancing enabled, the router sends one packet for 
destination1 over the first path, the second packet for (the same) destination1 over the second path, and 
so on. Per-packet load balancing ensures balancing over multiple links. 

Although path utilization with per-packet load balancing is beneficial, packets for a given pair of 
source-destination hosts might take different paths. This means that per-packet load balancing can 
introduce reordering of packets. This load balancing method would be inappropriate for certain types of 
data traffic (such as voice traffic over IP) that depend on packets arriving at the destination in sequence.

Use per-packet load balancing to ensure that a path for a single source-destination pair does not get 
overloaded. If the bulk of data passing through parallel links is for a single pair, per-destination load 
balancing overloads a single link while other links have very little traffic. Enabling per-packet load 
balancing allows you to use alternate paths to the same busy destination.

The Cisco 10000 series and Cisco 12000 series Internet Router are configured by default to perform 
per-destination load balancing.

On most Cisco IOS platforms, you need to separately configure load balancing for a destination IP prefix 
on each outgoing interface. Outgoing packets are then forwarded to the specified destination using the 
load balancing method that you configure.

Because of the Cisco 12000 series Internet router’s distributed architecture, you configure load 
balancing on a per line card basis. This means that all traffic passing through the outgoing interfaces of 
a line card is switched with either per-packet or per-destination load balancing.