How does an IGMP-enabled S series switch determine whether a member host exists in the multicast group

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In IGMPv1 on S series switches, hosts leave their multicast groups without sending Leave messages. The device uses IGMP group entries timer to determine the whether all multicast group members in the network segment have left.
In IGMPv2 and IGMPv3, hosts send Leave messages when leaving their group. After receiving the Done message, the querier sends a Group-Specific or a Source-Specific Query message to the network segment. The destination address of the Query message is the address of the multicast group and the group address in the message is also filled in with the address of the multicast group.
If other members of the group exist on the network segment, they respond with Report messages.
If no response is received when the timeout period ends, the querier considers that no member of the group exists on the network segment and cancels forwarding multicast data to the group.

Other related questions:
Why is no information about members of an IGMP multicast group displayed in the display igmp group command output on a CE series switch

If the display igmp group command output does not display any information, the specified multicast group has no members.

In normal cases, after users join a multicast group dynamically by sending IGMP Report messages, the IGMP multicast group information is displayed in the display igmp group command output. The following is an example:
<HUAWEI> display igmp group
Interface group report information of VPN instance: public net
 Vlanif100(10.1.6.2):
  Total 1 IGMP Group reported
   Group Address   Last Reporter   Uptime      Expires
   225.1.1.2       10.1.6.10       00:02:04    00:01:17
In the command output, Group Address indicates the IP address of the multicast group that users have joined, and Last Reporter indicates the last host that has sent an IGMP Report message.


How IGMP messages are transparently transmitted on an S series switch enabled with IGMP snooping
After global IGMP snooping is enabled on a switch, the switch sends all the IGMP messages received from a VLAN to the CPU for processing. If the multicast service is not configured in a VLAN and the switch needs to directly forward the IGMP messages of this VLAN without sending them to the CPU, run the protocol-transparent command in the VLAN view to enable transparent transmission of protocol packets. S series fixed switches support transparent transmission of protocol packets starting from V200R001, and only the S3700HI, S5710EI, S5720EI, S5700HI, S5710HI, S5720HI, S6700EI, andS6720EI support this function. S series modular switches support this function.

How can I check whether a loop exists on an S series switch
The simplest way to check the existence of a loop on an S series switch (except the S1700) is to check whether a MAC address flapping alarm exists on the switch. If the alarm exists, you can quickly locate the loop based on the information in the alarm. Run the display mac-address flapping record command to check whether MAC address flapping exists, or run the display trapbuffer command to check whether the alarm contains The mac-address has flap value or MAC move detected. For more information, visit Layer 2 Loop Troubleshooting.

Does STP need to be enabled if no loop exists on an S series switch
For S (except the S1700) series switches, STP is used to prevent loops on a network. If no loop exists, STP can be disabled.

Does an S series switch enabled with IGMP snooping querier participate in IGMP querier election
After IGMP snooping querier is enabled on an S series switch, the switch periodically sends Query messages to users in a VLAN but does not participate in IGMP querier election or forward Query messages to a VLAN. Layer 2 multicast querier is used when the switch connects to no upstream Layer 3 device or when the IGMP querier function is not enabled on the upstream Layer 3 device. If an IGMP querier exists on the network, do not enable the IGMP snooping querier function on the switch. This is because this function affects querier election on the upstream Layer 3 device and causes multicast service failures.

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