Cisco Group Management Protocol (CGMP) limits the forwarding of IP multicast packets to only those ports associated with IP multicast clients. These clients automatically join and leave groups that receive IP multicast traffic, and the switch dynamically changes its forwarding behavior according to these requests. CGMP provides the following services:

• Allows IP multicast packets to be switched only to those ports that have IP multicast clients.
• Saves network bandwidth on user segments by not propagating unnecessary IP multicast traffic.
• Does not require changes to the end host systems.
• Does not incur the overhead of creating a separate VLAN for each multicast group in the switched network.

When CGMP is enabled, it automatically identifies the ports to which the CGMP-capable router is attached. CGMP is enabled by default and supports a maximum of 64 IP multicast group registrations. Multicast routers that support CGMP periodically send CGMP join messages to advertise themselves to switches within a network. A receiving switch saves the information and sets a timer equal to the router hold time. The timer is updated every time the switch receives a CGMP join message advertising itself. When the last router hold time expires, the switch removes all IP multicast groups learned from CGMP.

CGMP works in conjunction with IGMP messages to dynamically configure Cisco Catalyst switch ports so that IP multicast traffic is forwarded only to those ports associated with IP multicast hosts. A CGMP-capable IP multicast router sees all IGMP packets and therefore can inform the Catalyst switches when specific hosts join or leave IP multicast groups. When the CGMP-capable router receives an IGMP control packet, it creates a CGMP packet that contains the request type (either join or leave), the multicast group address, and the actual MAC address of the host. The router then sends the CGMP packet to a well-known address to which all Catalyst switches listen. When a switch receives the CGMP packet, the switch interprets the packet and modifies the forwarding behavior of the multicast group. From then on, this multicast traffic is sent only to ports associated with the appropriate IP multicast clients. This process is done automatically, without user intervention.

Protocol Structure

• Count: Unsigned 8 bit integer
• Group Destination Address: The hardware MAC address of the destination device.
• Type: Message Type
• Unicast Source Address: The hardware MAC address of the unicast source device
• Version: CGMP version number

Related Terms: IPv4, IGMP, PIM-SM, RGMP

Sponsor Source:CGMP is a Cisco protocol.

Reference Links: http://www.cisco.com/: Cisco Group Management Protocol