Graco GE-DS-242-POE Manual De Usuario

 

Chapter 4: Web-Based Management 

GE-DS-242-PoE Managed Ethernet Switch User Manual 

93 

Multicast groups allow members to join or leave at any time. IGMP provides the 
method for members and multicast routers to communicate when joining or leaving 
a multicast group. 
IGMP version 1 is defined in RFC 1112. It has a fixed packet size and no optional data. 
The format of an IGMP packet is shown below: 

 

IGMP Message Format 
Octets 
     0   

 

 

 8                             16                    

 

 

                                31 

Type Response 

Time 

Checksum 

Group Address (all zeros if this is a query). 

 
The IGMP Type codes are shown below: 
 

0x11 

Membership Query (if Group Address is 0.0.0.0). 

0x11 

Specific Group Membership Query (if Group Address is Present). 

0x16 

Membership Report (version 2). 

0x17 

Leave a Group (version 2). 

0x12 

Membership Report (version 1). 

 
IGMP packets enable multicast routers to keep track of the membership of multicast 
groups, on their respective sub networks. The following outlines what is 
communicated between a multicast router and a multicast group member using 
IGMP. 
A host sends an IGMP "report" to join a group. 
A host will never send a report when it wants to leave a group (for version 1). 
A host will send a "leave" report when it wants to leave a group (for version 2). 
Multicast routers send IGMP queries (to the all-hosts group address: 224.0.0.1) 
periodically to see whether any group members exist on their sub networks. If there is 
no response from a particular group, the router assumes that there are no group 
members on the network.