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If a Layer Two switch doesn't have the capabilities to run IGMP Snooping, it will be capable to run CGMP - Cisco Group Membership Protocol. CGMP makes it possible for the multicast router to work with the Layer Two switch to eradicate unnecessary multicast forwarding.
CGMP will be enabled on each the multicast router and the switch, but the router's going to do all the operate. The router will be sending Join and Leave messages to the switch as necessary. PIM need to be operating on the router interface facing the switch just before enabling CGMP, as you can see:
R1(config)#int e0
R1(config-if)#ip cgmp
WARNING: CGMP needs PIM enabled on interface
R1(config-if)#ip pim sparse
R1(config-if)#ip cgmp
When CGMP is 1st enabled on both the multicast router and switch, the router will send a CGMP Join message, informing the switch that a multicast router is now connected to it. This specific CGMP Join will include a Group Location Address (GDA) of 0000.0000.0000 and the MAC address of the sending interface. The GDA is utilized to recognize the multicast group, so when this is set to all zeroes, the switch knows this is an introductory CGMP Join, letting the switch know that the multicast router is on line.
The switch makes an entry in its MAC table that this router can be identified off the port that the CGMP Join came in on. The router will send a CGMP Join to the switch just about every minute to serve as a keepalive.
A workstation connected to the switch on port /five now wishes to join multicast group 225.1.1.1. The Join message is sent to the multicast router, but initial it will pass by means of the switch. The switch will do what you'd anticipate it to do - study the source MAC address and make an entry for it in the MAC address table as getting off port quick /5 if there is not an entry already there. (Don't forget that the MAC address table is also referred to as the CAM table or the bridging table.)
The router will then obtain the Join request, and send a CGMP Join back to the switch. This CGMP Join will include each the multicast group's MAC address and the requesting host's MAC address. Now the switch knows about the multicast group 225.1.1.1 and that a member of that group is identified off port fast /five. In the future, when the switch receives frames destined for that multicast group, the switch will not flood the frame as it would an unknown multicast. As an alternative, the switch will forward a copy of the frame to every port that it knows leads to a member of the multicast group.
Two major added benefits of CGMP are the explicit Join and Leave Group messages. In the next aspect of this BCMSN exam tutorial, properly take a look at the Leave Group messages. site
