3Com 10014299 manual Igmp Overview, Configuring Igmp, Displaying and Debugging Igmp

34 CONFIGURING IGMP

	This chapter covers the following topics:
	■	IGMP Overview
	■	Configuring IGMP
	■ Displaying and Debugging IGMP
	■	IGMP Configuration Example
IGMP Overview	The Internet Group Management Protocol (IGMP) is a protocol that is responsible
	for the IP multicast member management among the TCP/IP protocol family. It is

the basis for IP multicast, and it is used to establish and maintain multicast membership between the IP hosts and the multicast routers directly adjacent to the hosts. IGMP does not include the propagation and maintenance of the membership relationship information between multicast routers, which is accomplished by each multicast routing protocol. IGMP operates on a physical network, such as a single Ethernet segment.

At present, IGMP Version 1 and IGMP Version 2 are extensively used. IGMP Version

2 specifies the following three kinds of messages:

■ Membership Query Message: According to different group addresses, it can be classified into a general query message or a group-specific query message, used to learn if a particular group has any members attached on a network. For a group-specific query message, the router is used to check whether there is any subscriber in a connecting network who wants to make the query message valid, and the target group address must be zero or a valid multicast group address. IGMP Version 2 allows routers to send group-specific query messages.

■ Membership Report Message: When the host receives a general query or a group-specific membership query message, it first identifies the combination with the interface sending the query message and sets a host group delay timer for each member group. If the remaining time of this timer is larger than the maximum response time set in the query message, it is changed to the maximum response time value. The host broadcasts the membership report to this router before the time runs out. Once the router receives the membership report, it adds the group to the membership list of the network it belongs to, and starts the group membership interval timer. If the router does not receive any membership report with the maximum query response timeout, it becomes clear that there is no local group member, and it does not transmit the received multicast message to the network it connects to.

■ Leave Group Message: IGMP Version 2 allows a host to send a leave group message to all routers when it leaves a multicast group (the target group address is 224.0.0.2).