--> By default Switch will forward the frame to all the interfaces in Broadcast Domain or VLAN whenever it receives unknown destination multicast MAC address.
--> There are 2 methods used by switches to know how to send multicast traffic to particular hosts.
1) CGMP -- (Cisco Proprietary)
2) IGMP Snooping ( Open Standard)
CGMP:
--> Router knows all the Multicast MAC addresses as well as Multicast group addresses used by Hosts with the help of IGMP messages.
--> By using CGMP we enable the router to send this information to Switch so that it can add this information in CAM Table of the Switch.
--> Only Routers can send CGMP messages whereas Switches will listen for CGMP messages.
--> We can enable CGMP only on Layer 3 Interface with the help of following command.
Router(config)# interface fa0/1
Router(config-if)# ip cgmp
--> Main Problem with CGMP is it will work with only Cisco Switches, For other switches we have to use IGMP snooping.
IGMP Snooping:
--> IGMP Snooping used in multi vendor network to send multicast traffic to particular hosts.
--> There are 2 methods used by switches to know how to send multicast traffic to particular hosts.
1) CGMP -- (Cisco Proprietary)
2) IGMP Snooping ( Open Standard)
CGMP:
--> Router knows all the Multicast MAC addresses as well as Multicast group addresses used by Hosts with the help of IGMP messages.
--> By using CGMP we enable the router to send this information to Switch so that it can add this information in CAM Table of the Switch.
--> Only Routers can send CGMP messages whereas Switches will listen for CGMP messages.
--> We can enable CGMP only on Layer 3 Interface with the help of following command.
Router(config)# interface fa0/1
Router(config-if)# ip cgmp
--> Main Problem with CGMP is it will work with only Cisco Switches, For other switches we have to use IGMP snooping.
IGMP Snooping:
--> IGMP Snooping used in multi vendor network to send multicast traffic to particular hosts.
--> When IGMP Snooping is enabled on the Switch, It will listen all IGMP messages which are going between Host and Router.
In order to understand how IGMP snooping is working , you need to understand two MAC addresses
1) USA ( Unicast Source Address)
2) GDA ( Group Destination Address)
1) When a Host want to join any group, it is going to send Report Message to the Local Router. This message goes to the Router via Switch
Destination MAC address --- Group Multicast MAC address (0100-5EXX-XXXX) (GDA)
Source MAC address --------- Host MAC address (USA)
2) Switch Receives this IGMP Report message, Checks for GDA in CAM Table, If it is not present then it will create GDA( Group Multicast MAC) in CAM Table and adds Host MAC address as well as Router Port MAC address into CAM Table.
3) When Switch Receives IGMP Leave Message, it will remove the source mac address from the CAM table associated with Group Multicast MAC.By this switch will save bandwidth
Note: By default IGMP Snooping is enabled in all switches
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