3Com 10014298 manual Implementing Igmp Snooping

126CHAPTER 6: MULTICAST PROTOCOL

Figure 30 Implementing IGMP Snooping

Internet

A router running

IGMP

IGMP packets

An Ethernet switch running IGMP snooping

IGMP packets

1IGMP 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.

2IGMP 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.

3IGMP 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.

4IGMP 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