Hardware And Software Forwarding - Cisco Catalyst 4500 Series Software Configuration Manual

Chapter 38 Configuring IP Multicast

Hardware and Software Forwarding

The Integrated Switching Engine forwards the majority of packets in hardware at very high rates of
speed. The CPU subsystem forwards exception packets in software. Statistical reports should show that
the Integrated Switching Engine is forwarding the vast majority of packets in hardware.
Figure 38-5
Figure 38-5
In the normal mode of operation, the Integrated Switching Engine performs inter-VLAN routing in
hardware. The CPU subsystem supports generic routing encapsulation (GRE) tunnels for forwarding in
software.
Replication is a particular type of forwarding where, instead of sending out one copy of the packet, the
packet is replicated and multiple copies of the packet are sent out. At Layer 3, replication occurs only
for multicast packets; unicast packets are never replicated to multiple Layer 3 interfaces. In IP
multicasting, for each incoming IP multicast packet that is received, many replicas of the packet are sent
out.
IP multicast packets can be transmitted on the following types of routes:
•
•
•
Hardware routes occur when the Integrated Switching Engine hardware forwards all replicas of a packet.
Software routes occur when the CPU subsystem software forwards all replicas of a packet. Partial routes
occur when the Integrated Switching Engine forwards some of the replicas in hardware and the CPU
subsystem forwards some of the replicas in software.
Partial Routes
The conditions listed below cause the replicas to be forwarded by the CPU subsystem software, but the
Note
performance of the replicas that are forwarded in hardware is not affected.
shows a logical view of the hardware and software forwarding components.
Hardware and Software Forwarding Components
Integrated Switching Engine
L3 physical
interface
Gig 1/1
L3 interfaces
VLAN1 VLAN2
L2 switchports
Hardware routes
Software routes
Partial routes
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
CPU Subsystem
Router
GRE
tunnel
About IP Multicast
GRE
tunnel
38-9