Cisco Cisco IOS Software Release 12.0(21)S
Per-Packet Load Balancing
Feature Overview
2
Cisco IOS Release: Multiple releases (see the Feature History table)
Feature Overview
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.
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
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.
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 and Per-Packet
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.
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.
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.
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.
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.
Note
The Cisco 10000 series and Cisco 12000 series Internet Router are configured by default to perform
per-destination load balancing.
per-destination load balancing.
Implementing Per-Packet 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.
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.
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.