3Com Switch 8800 Configuration Guide
Chapter 24 IGMP Snooping Configuration
24-3
II. Implement Layer 2 multicast with IGMP Snooping
The Ethernet switch runs IGMP Snooping to listen to the IGMP messages and map the
host and its ports to the corresponding multicast group address. To implement IGMP
Snooping, the Layer 2 Ethernet switch processes different IGMP messages in the way
illustrated in the figure below:
Internet
IGMP packets
A Ethernet Switch
running IGMP Snooping
A
router
running IGMP
IGMP packets
Internet
IGMP packets
A Ethernet switch
running IGMP Snooping
A
router
running IGMP
IGMP packets
Figure 24-3
Implement IGMP Snooping
z
IGMP general query message: Transmitted by the multicast router to the multicast
group members to query which multicast group contains member. When an IGMP
general query message arrives at a router port, the Ethernet switch will reset the
aging timer of the port. When a port other than a router port receives the IGMP
general query message, the Ethernet switch will notify the multicast router that a
port is ready to join a multicast group and starts the aging timer for the port.
z
IGMP specific query message: Transmitted from the multicast router to the
multicast members and used for querying if a specific group contains any member.
When received IGMP specific query message, the switch only transmits the
specific query message to the IP multicast group which is queried.
z
IGMP report message: Transmitted from the host to the multicast router and used
for applying for joining a multicast group or responding to the IGMP query
message. When received the IGMP report message, the switch checks if the MAC
multicast group, corresponding to the IP multicast group the packet is ready to join
exists.
If the corresponding MAC multicast group does not exist, the switch only notifies the
router that a member is ready to join a multicast group, creates a new MAC multicast
group, adds the port received the message to the group, starts the port aging timer, and
then adds all the router ports in the native VLAN of the port into the MAC multicast
