Information About Ip Multicast In A Switched Ethernet Network; Ip Multicast Traffic And Layer 2 Switches; Cgmp On Catalyst Switches For Ip Multicast - Cisco Catalyst 3650 Configuration Manual

Information About IP Multicast in a Switched Ethernet Network

Information About IP Multicast in a Switched Ethernet Network

IP Multicast Traffic and Layer 2 Switches

The default behavior for a Layer 2 switch is to forward all multicast traffic to every port that belongs to the
destination LAN on the switch. This behavior reduces the efficiency of the switch, whose purpose is to limit
traffic to the ports that need to receive the data. This behavior requires a constraining mechanism to reduce
unnecessary multicast traffic, which improves switch performance.
Cisco Group Management Protocol (CGMP), Router Group Management Protocol (RGMP), and IGMP
snooping efficiently constrain IP multicast in a Layer 2 switching environment.
• CGMP and IGMP snooping are used on subnets that include end users or receiver clients.
• RGMP is used on routed segments that contain only routers, such as in a collapsed backbone.
• RGMP and CGMP cannot interoperate. However, Internet Group Management Protocol (IGMP) can

CGMP on Catalyst Switches for IP Multicast

CGMP is a Cisco-developed protocol used on device connected to Catalyst switches to perform tasks similar
to those performed by IGMP. CGMP is necessary for those Catalyst switches that do not distinguish between
IP multicast data packets and IGMP report messages, both of which are addressed to the same group address
at the MAC level. The switch can distinguish IGMP packets, but would need to use software on the switch,
greatly impacting its performance.
You must configure CGMP on the multicast device and the Layer 2 switches. The result is that, with CGMP,
IP multicast traffic is delivered only to those Catalyst switch ports that are attached to interested receivers.
All other ports that have not explicitly requested the traffic will not receive it unless these ports are connected
to a multicast router. Multicast router ports must receive every IP multicast data packet.
Using CGMP, when a host joins a multicast group, it multicasts an unsolicited IGMP membership report
message to the target group. The IGMP report is passed through the switch to the router for normal IGMP
processing. The router (which must have CGMP enabled on this interface) receives the IGMP report and
processes it as it normally would, but also creates a CGMP Join message and sends it to the switch. The Join
message includes the MAC address of the end station and the MAC address of the group it has joined.
The switch receives this CGMP Join message and then adds the port to its content-addressable memory (CAM)
table for that multicast group. All subsequent traffic directed to this multicast group is then forwarded out the
port for that host.
The Layer 2 switches are designed so that several destination MAC addresses could be assigned to a single
physical port. This design allows switches to be connected in a hierarchy and also allows many multicast
destination addresses to be forwarded out a single port.
The device port also is added to the entry for the multicast group. Multicast device must listen to all multicast
traffic for every group because IGMP control messages are also sent as multicast traffic. The rest of the
multicast traffic is forwarded using the CAM table with the new entries created by CGMP.
Related Topics
Enabling CGMP, on page 104
IP Multicast Routing Configuration Guide, Cisco IOS XE Release 3SE (Catalyst 3650 Switches)
102
interoperate with CGMP and RGMP snooping.
Constraining IP Multicast in Switched Ethernet
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