CGMP: Cisco Group Management Protocol

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 groupin  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 groupregistrations.  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 - CGMP: Cisco Group Management Protocol

1 byte	6 bytes	1 byte	6 bytes	1 byte
Count	Group Destination Address	Type	Unicast Source Address	Version

• 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

CDP is a Cisco protocol.CGMP is a Cisco protocol.

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