Cisco Systems 4500 CEF Configuration Restrictions, Configuring CEF

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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 31 Configuring Cisco Express Forwarding
CEF Configuration Restrictions
Load Balancing
The Catalyst 4500 series switch supports load balancing for routing packets in the integrated switching
engine hardware. Load balancing is always enabled. It works when multiple routes for the same network
with different next-hop addresses are configured. These routes can be configured either statically or
through a routing protocol such as OSPF or EIGRP.
The hardware makes a forwarding decision by using a hardware load sharing hash function to compute
a value, based on the source and destination IP addresses and the source and destination TCP port
numbers (if available). This load sharing hash value is then used to select which route to use to forward
the packet. All hardware switching within a particular flow (such as a TCP connection) is routed to the
same next hop, which reduces the chance that packet reordering occurs. Up to eight different routes for
a particular network are supported.
Software Interfaces
Cisco IOS for the Catalyst 4500 series switch supports GRE and IP tunnel interfaces that are not part of
the hardware forwarding engine. All packets that flow to or from these interfaces must be processed in
software and have a significantly lower forwarding rate than that of hardware-switched interfaces. Also,
Layer 2 features are not supported on these interfaces.
CEF Configuration Restrictions
The CEF integrated switching engine supports only ARPA and ISL/802.1q encapsulation types for Layer
3 switching in hardware. The CPU subsystem supports a number of encapsulations such as SNAP for
Layer 2 switching that you can use for Layer 3 switching in software.
Configuring CEF
These sections describe how to configure CEF:
• Enabling CEF, page 31-6
• Configuring Load Balancing for CEF, page 31-7
Enabling CEF
By default, CEF is enabled globally on the Catalyst 4500 series switch. No configuration is required.
To reenable CEF, perform this task:
Command Purpose
Switch(config)# ip cef distributed Enables standard CEF operation.