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Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 33 Configuring IP Multicast
About 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 33-5 shows a logical view of the hardware and software forwarding components.
Figure 33-5 Hardware and Software Forwarding Components
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
• Software routes
• Partial 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
Note The conditions listed below cause the replicas to be forwarded by the CPU subsystem software, but the
performance of the replicas that are forwarded in hardware is not affected.
Integrated Switching Engine
CPU Subsystem
GRE
tunnel
L3 physical
interface
Router
L3 interfaces
L2 switchports
68127
VLAN1
Gig 1/1
VLAN2
GRE
tunnel