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126 CHAPTER 6: MULTICAST PROTOCOL
Figure 30 Implementing IGMP Snooping
1 IGMP general query message: Transmitted by the multicast router to query which
multicast group contains member. When a router port receives an IGMP general
query message, the Switch 7750 will reset the aging timer of the port. When a
port other than a router port receives the IGMP general query message, the Switch
7750 will notify the multicast router that a port is ready to join a multicast group
and starts the aging timer for the port.
2 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.
3 IGMP report message: Transmitted from the host to the multicast router and used
for applying to a multicast group or responding to the IGMP query message.
When received, the switch checks if the MAC multicast group is ready to join. If
the corresponding MAC multicast group does not exist, the switch notifies the
router that a member is ready to join a multicast group, creates a new MAC
multicast group, adds the port that 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 forwarding table. Meanwhile, it creates an IP multicast
group and adds the port received to it. If the corresponding MAC multicast group
exists but does not contain the port that received the report message, the switch
adds the port into the multicast group and starts the port aging timer. Then, the
switch checks if the corresponding IP multicast group exists. If it does not exist, the
switch creates a new IP multicast group and adds the port that received the report
message to it. If it does exist, the switch adds the port. If the corresponding MAC
multicast group exists and contains the port, the switch will only reset the aging
timer of the port.
4 IGMP leave message: Transmitted from the multicast group member to the
multicast router, to notify that a host has left the multicast group. The Switch
7750 transmits the specific query message, concerning the group, to the port that
received the message in an effort to check if the host still has other members of
this group, and then starts a maximum response timer. If the switch has not
received any report message from the multicast group, the port will be removed
from the corresponding MAC multicast group. If the MAC multicast group does
Internet
A router running
IGMP
An Ethernet switch
running IGMP
snooping
IGMP packets
IGMP packets