3com DUA1750-2BAA01 Manual De Usuario

3Com Switch 8800 Configuration Guide 

Chapter 27  IGMP Configuration

 

27-1 

Internet Group Management Protocol (IGMP) is a protocol in the TCP/IP suite 

responsible for management of IP multicast members. It is used to establish and 

maintain multicast membership among IP hosts and their directly connected 

neighboring routers. IGMP excludes transmitting and maintenance of membership 

information among multicast routers, which are completed by multicast routing 

protocols. All hosts participating in multicast must implement IGMP. 

Hosts participating in IP multicast can join and leave a multicast group at any time. The 

number of members of a multicast group can be any integer and the location of them 

can be anywhere. A multicast router does not need and cannot keep the membership of 

all hosts. It only uses IGMP to learn whether receivers (i.e., group members) of a 

multicast group are present on the subnet connected to each interface. A host only 

needs to keep which multicast groups it has joined.  

IGMP is not symmetric on hosts and routers. Hosts need to respond to IGMP query 

messages from the multicast router, i.e., report the group membership to the router. 

The router needs to send membership query messages periodically to discover 

whether hosts join the specified group on its subnets according to the received 

response messages. When the router receives the report that hosts leave the group, 

the router will send a group-specific query packet (IGMP Version 2) to discover whether 

no member exists in the group. 

Up to now, IGMP has three versions, namely, IGMP Version 1 (defined by RFC1112), 

IGMP Version 2 (defined by RFC2236) and IGMP Version 3. At present, IGMP Version 

2 is the most widely used version. 

IGMP Version 2 boasts the following improvements over IGMP Version 1: 

A shared network segment means that there are multiple multicast routers on a network 

segment. In this case, all routers running IGMP on the network segment can receive 

the membership report from hosts. Therefore, only one router is necessary to send 

membership query messages. In this case, the router election mechanism is required 

to specify a router as the querier.  

In IGMP Version 1, selection of the querier is determined by the multicast routing 

protocol. While IGMP Version 2 specifies that the multicast router with the lowest IP