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Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter24 Understanding and Configuring IP Multicast
Overview of IP Multicast
Output interface lists are stored in the multicast expansion table (MET). The MET has room for up to
32,000 output interface lists. The MET resources are shared by both Layer 3 multicast routes and by
Layer 2 multicast entries. The actual number of output interface lists available in hardware depends on
the specific configuration. If the total number of multicast routes exceed 32,000, multicast packets might
not be switched by the Integrated Switching Engine. They would be forwarded by the CPU subsystem
at much slower speeds.
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 24-5 shows a logical view of the hardware and software forwarding components.
Figure24-5 Hardware and Software Forwarding Components
In the normal mode of operation, the Integrated Switching Engine performs inter-VLA N routing in
hardware. The CPU subsystem supports generic routing encapsulation (GRE) tunnels for forwar ding 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
Integrated Switching Engine CPU Subsystem
GRE
tunnel
L3 physical
interface
Router
L3 interfaces
L2 switchports
68127
VLAN1
Gig 1/1
VLAN2 GRE
tunnel