PLANET FGSW-2840 - Manual (Page 140)

User’s Manual of FGSW-2840 / FGSW-4840S
4.6.1 IGMP Snooping
IGMP Snooping Process
The Managed Switch running IGMP Snooping, listens to the IGMP messages transmitted between the host and the router, and
tracks the IGMP messages and the registered port. When receiving IGMP report message, the Managed Switch adds the port to
the multicast address table; when the Managed Switch listens to IGMP leave message from the host, the router sends the
Group-Specific Query message of the port to check if other hosts need this multicast, if yes, the router will receive IGMP report
message; if no, the router will receive no response from the hosts and the Managed Switch will remove the port from the
multicast address table. The router regularly sends IGMP query messages. After receiving the IGMP query messages, the
Managed Switch will remove the port from the multicast address table if the Managed Switch receives no IGMP report message
from the host within a period of time.
IGMP Messages
The Managed Switch running IGMP Snooping processes the IGMP messages of different types as follows.
1. IGMP Query Message
IGMP query message, sent by the router, falls into two types, IGMP general query message and IGMP group-specific-query
message. The router regularly sends IGMP general message to query if the multicast groups contain any member. When
receiving IGMP leave message, the receiving port of the router will send IGMP group-specific-query message to the multicast
group and the Managed Switch will forward IGMP group-specific-query message to check if other members in the multicast
group of the port need this multicast.
When receiving IGMP general query message, the Managed Switch will forward them to all other ports in the VLAN owning the
receiving port. The receiving port will be processed: if the receiving port is not a router port yet, it will be added to the router port
list with its router port time specified; if the receiving port is already a router port, its router port time will be directly reset.
When receiving IGMP group-specific-query message, the Managed Switch will send the group-specific query message to the
members of the multicast group being queried.
2. IGMP Report Message
IGMP report message is sent by the host when it applies for joining a multicast group or responses to the IGMP query message
from the router.
When receiving IGMP report message, the Managed Switch will send the report message via the router port in the VLAN as well
as analyze the message to get the address of the multicast group the host applies for joining. The receiving port will be
processed: if the receiving port is a new member port, it will be added to the multicast address table with its member port time
specified; if the receiving port is already a member port, its member port time will be directly reset.
3. IGMP Leave Message
The host, running IGMPv1, does not send IGMP leave message when leaving a multicast group, as a result, the Managed
Switch can not get the leave information of the host momentarily. However, after leaving the multicast group, the host does not
send IGMP report message any more, so the Managed Switch will remove the port from the corresponding multicast address
table when its member port time times out. The host, running IGMPv2 or IGMPv3, sends IGMP leave message when leaving a
multicast group to inform the multicast router of its leaving.
When receiving IGMP leave message, the Managed Switch will forward IGMP group-specific-query message to check if other
members in the multicast group of the port need this multicast and reset the member port time to the leave time. When the leave
time times out, the Managed Switch will remove the port from the corresponding multicast group. If no other member is in the
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