IGMP Snooping Leave Process Between the Source and Multiple Receivers on the Same VLAN

Figure 9-7 and the subsequent steps outlined address the issue of a receiver leaving a multicast group while another receiver on the same VLAN is still interested in the traffic.

Step 1. Host2 sends a leave group message to all routers, 224.0.0.2, for multicast traffic 239.1.1.1.

Step 2. The Catalyst switch intercepts the IGMP leave message:

MCAST-IGMPQ:recvd an IGMP Leave on the port 10/1 vlanNo 3 GDA 239.1.1.1

Step 3. The switch sends a MAC-based general query to port 10/1. A random deletion timer value (1 to 3 seconds) will be set. If the host is communicating via IGMPv1, the random deletion timer is set at 10 seconds:

MCAST-DEL-TIMER: Deletion Timer Value set to Random Value 3

Step 4. If the switch does not hear a membership report during the random deletion timer, it will drop the port from its multicast forwarding table:

MCAST-TIMER:IGMPLeaveTimer expired on port 10/1 vlanNo 3 GDA 01-00-5e-

  01-01-01

Delete UpdatePortOnMulticast

Step 5. The switch does not send the IGMP leave message to the MSFC because the switch knows that Host3, which is on the same VLAN as Host2, is still accepting the 239.1.1.1 traffic. The MSFC does not even know that Host2 has left the multicast group. Also, no changes have been made to the MMLS table because Host3 on the VLAN 3 is still accepting the traffic.