/mcpIGMP on a Router If, by default, there is no user registered to a specific group in a subnet, the router does not forward multicast traffic for that group into that subnet. That means that a router needs to receive an IGMP report for a GDA in order to add it to the multicast routing table and to start forwarding traffic for that group. On a router, you need to perform the following actions: Enable multicast routing in the global mode, as shown below. ip multicast-routing 1. Configure a multicast routing protocol on the involved interface, as shown below. ip pim dense-mode 2. Monitor IGMP, as shown below. show ip igmp interface show ip igmp group show ip mroute 3. Configure a router to send the IGMP report (on the interface), as shown below. ip igmp join-group [GDA_ip_address] ip igmp version [1 | 2 | 3] 4. Practical Example on a Router A router is configured to route between two subinterfaces, Fast-Ethernet 0.2 and Fast-Ethernet 0.3. Both interfaces are also configured to run IGMP. In the output below, you can see the IGMP version, the group joined, and so on. Configuration ip multicast-routing interface FastEthernet0 no ip address no ip directed-broadcast ! interface FastEthernet0.2 encapsulation isl 2 ip address 10.2.2.1 255.255.255.0 no ip redirects no ip directed-broadcast ip pim dense-mode ! interface FastEthernet0.3 encapsulation isl 3 ip address 10.3.3.1 255.255.255.0