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Chapter 3 - Internet Group Management Protocol

Cisco Multicast Routing & Switching
William R. Parkhurst
  Copyright © 1999 The McGraw-Hill Companies

Internet Group Management Protocol, IGMP Version 2
IGMP version 2 is detailed in RFC 2236 (Copyright © The Internet Society 1997), written by W. Fenner of Xerox PARC in November 1997. IGMP version 2 messages have the format shown in Figure 3-11. The shaded parameters highlight the changes from the IGMP version 1 packet.
Figure 3-11: IGMP version 2 packet format
Type:
0x11 = Membership Query
0x16 = Version 2 Membership Report
0x17 = Leave Group
0x12 = Version 1 Membership Report for backwards
             compatibility with IGMP version 1.
Membership Query messages, type 0x11, come in two flavors. The first is a General Query that is used to determine which groups on a network have active members. The second is a Group-Specific Query that is used to determine if a particular multicast group has active members. The type of Membership Query message can be determined by the group address. For a General Query, the group address is zero and, for a Membership Query, the group address contains the address of the multicast group that is being queried.
The Maximum Response Time field (Max. Rtime) applies only to Membership Query messages. This field specifies the maximum amount of time a host can wait before responding to a Membership Query report. Maximum Response Time is in units of 0.1 seconds.
Protocol Operation
One improvement that IGMP version 2 has over version 1 concerns multi-access networks, such as ethernet, that have more than one attached multicast router (see Figure 3-12). Only one router needs to send Membership Query messages because all attached routers running IGMP hear the Membership Report messages from the hosts. IGMP version 2 adds a feature that enables routers to determine which router is responsible for sending Membership Query messages with the other routers becoming Non-Querier routers. In Figure 3-12, assume that router A sends the first Membership Query message onto the multiaccess network. Router B receives this message and, because router A has a lower IP address than router B, router A remains the Querier for the network and router B the Non-Querier. If router B had sent the first Membership Query message (all routers start in the role of Querier), this would not suppress Membership Query messages from router A because router A has the lower IP address. Router A would send a Membership Query message and router B, upon receiving this message, would become a Non-Querier for the network.
Figure 3-12: On a multi-access network, the router with the lowest IP address becomes the Querier.

 


 
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