Filter
Top Products
24-4
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter 24 Understanding and Configuring IP Multicast
Overview of IP Multicast
IGMP Snooping and CGMP
IGMP snooping is used for multicasting in a Layer 2 switching environment. With IGMP snooping, a
Layer 3 switch or router examines Layer 3 information in the IGMP packets in transit between hosts and
a router. When the switch receives the IGMP Host Report from a host for a particular multicast group,
the switch adds the host's port number to the associated multicast table entry. When the switch receives
the IGMP Leave Group message from a host, it removes the host's port from the table entry.
Because IGMP control messages are transmitted as multicast packets, they are indistinguishable from
multicast data if only the Layer 2 header is examined. A switch running IGMP snooping examines every
multicast data packet to determine whether it contains any pertinent IGMP control information. If IGMP
snooping is implemented on a low end switch with a slow CPU, performance could be severely impacted
when data is transmitted at high rates. On the Catalyst 4500 series switches, IGMP snooping is
implemented in the forwarding ASIC, so it does not impact the forwarding rate.
Note A Catalyst 4500 series switch can act as a CGMP server for switches that do not support IGMP snooping,
such as Catalyst 4500 family switches with Supervisor Engine I and Supervisor Engine II. You cannot
configure the switch as a CGMP client. To configure a Catalyst 4500 series switch as a client, use IGMP
snooping.
CGMP is a Cisco protocol that allows Catalyst switches to leverage IGMP information on Cisco routers
to make Layer 2 forwarding decisions. CGMP is configured on the multicast routers and the Layer 2
switches. As a result, IP multicast traffic is delivered only to those Catalyst switchports with hosts that
have requested the traffic. Switchports that have not explicitly requested the traffic will not receive it.
IP Multicast on the Catalyst 4500 Series Switch
The Catalyst 4500 series switch supports an ASIC-based Integrated Switching Engine that provides
Ethernet bridging at Layer 2 and IP routing at Layer 3. Because the ASIC is specifically designed to
forward packets, the Integrated Switching Engine hardware provides very high performance with ACLs
and QoS enabled. At wire-speed, forwarding in hardware is significantly faster than the CPU subsystem
software, which is designed to handle exception packets.
The Integrated Switching Engine hardware supports interfaces for inter-VLAN routing and switchports
for Layer 2 bridging. It also provides a physical Layer 3 interface that can be configured to connect with
a host, a switch, or a router.
Figure 24-2 shows a logical view of Layer 2 and Layer 3 forwarding in the Integrated Switching Engine
hardware.