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Chapter 3 - Internet Group Management Protocol

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

Chapter 3: Internet Group Management Protocol
Overview
When a multicast router receives traffic destined for a multicast group, the router needs to know on which interfaces the traffic should be forwarded. The decision to forward is based on whether or not any group members or forwarding routers are on the subnet. Forwarding multicast traffic onto a subnet that has no group members is a waste of bandwidth.
Figure 3-1 illustrates the situation where a multicast router is receiving traffic for the group 224.65.10.154. Subnet 1 has no group members, so there is no need for the router to forward the traffic to subnet 1. Subnet 2 has one host, host C, which is a member of the multicast group 224.65.10.154, so the multicast traffic will be forwarded to subnet 2. What if host D in Figure 3-1 joins the group? The router only needs to know that at least one group member is on the subnet and it does not matter to the router if there is one group member or if there are 100.
Figure 3-1: Forwarding of multicast traffic
Figure 3-2 shows the scenario where subnet 1 has no group members, but a downstream multicast router on subnet 1 has group members attached to one of the router’s interfaces. The multicast traffic would need to be forwarded onto subnet 1. As shown, the Internet Group Management Protocol (IGMP) is used between hosts and routers, and the multicast routing protocols, Distance Vector Multicast Routing Protocol (DVMRP) and Protocol Independent Multicast (PIM), are used between multicast routers.
Figure 3-2: Forwarding of multicast traffic to a downstream multicast router
IGMP is the mechanism used by hosts on a network to inform directly-attached routers which multicast group(s) the host wants to either join or leave. Multicast routers use IGMP to determine if any members of the multicast groups are located on any of their attached networks. If group members are present, multicast routers can then join a particular multicast group and forward multicast traffic to hosts that have joined the group(s). The original IGMP specification is detailed in RFC 1112, “Host Extensions for IP Multicasting.” This specification is typically referred to as IGMP version 1 and was written by S. Deering of Stanford University in August 1989. A subsequent RFC, written by W. Fenner of Xerox PARC, updated the original IGMP version 1 RFC. The update is RFC 2236, “Internet Group Management Protocol, Version 2.” Both RFCs will be examined because a mix of IGMP version 1 and version 2 hosts and routers may be present in the network, and you need to be aware of interoperability issues between the versions. Following the discussion of IGMP version 1 and version 2, we will examine configuring, monitoring, and debugging IGMP on Cisco routers.

 


 
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