Cisco Catalyst 4500 Series, Software Configuration Manual

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Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 36 Configuring Cisco Express Forwarding
Configuring CEF
Configuring Load Balancing for CEF
CEF 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 can configure load balancing on a per-destination basis. Load-balancing decisions are made on the
outbound interface. You can configure per-destination load balancing for CEF on outbound interfaces.
The following topics are discussed:
• Configuring Per-Destination Load Balancing, page 36-7
•
Configuring Load Sharing Hash Function, page 36-7
• Viewing CEF Information, page 36-8
Configuring Per-Destination Load Balancing
Per-destination load balancing is enabled by default when you enable CEF. To use per-destination load
balancing, you do not perform any additional tasks once you enable CEF.
Per-destination load balancing allows the router to use 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. Traffic destined for different pairs tend to take different paths. Per-destination load balancing
is enabled by default when you enable CEF; it is the load balancing method of choice in most situations.
Because per-destination load balancing depends on the statistical distribution of traffic, load sharing
becomes more effective as the number of source-destination pairs increases.
Use per-destination load balancing to ensure that packets for a given host pair arrive in order. All packets
for a certain host pair are routed over the same link or links.
Configuring Load Sharing Hash Function
When multiple unicast routes exist to a particular destination IP prefix, the hardware sends packets
matching that prefix across all possible routes, which shares the load across all next hop routers. By
default, the route used is chosen by computing a hash of the source and destination IP addresses and
using the resulting value to select the route. This preserves packet ordering for packets within a flow by
ensuring that all packets within a single IP source/destination flow are sent on the same route, but it
provides a near-random distribution of flows to routes.
You can change the load-sharing hash function. So, in addition to the source and destination IP
addresses, the source TCP/UDP port, the destination TCP/UDP port, or both can also be included in the
hash.
To the configure load sharing hash function to use the source and/or destination ports, perform this task:
Command Purpose
Switch(config)# ip cef distributed
Enables standard CEF operation.