Internet Group Management Protocol

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.

RFC 1112, Host Extensions for IP Multicasting (IGMP Version 1)

RFC 1112 obsoletes RFCs 988 and 1054 and details the requirements of a host in order for it to be able to support IP multicasting. The multicasting support needed is for hosts to be able to join and leave multicast groups with IP addresses in the range 224.0.0.0 to 239.255.255.255. Also specified are the mechanisms for hosts to be able to receive and send multicast traffic.

A host can have one out of three levels of multicasting capabilities. Level 0 defines a host that has no multicasting functionality beyond being able to detect and discard an IP Class D multicast packet. A level 1 host can send but not receive IP multicast traffic, while a level 2 host is a fully capable multicast entity and can send and receive multicast traffic. Level 2 hosts are required to implement IGMP and we will assume that all hosts in the following discussion are level 2 hosts. The relationship between IGMP and IP layered models is shown in Figure 3-3.

Figure 3-3: IGMP resides at the network layer of the IP layered model.

Sending an IGMP packet is really no different than sending a broadcast or unicast IP packet, although additional functionality is required for a level 2 host. The first required function concerns the TTL field in the IGMP packet. If a TTL value is not explicitly set, then the default TTL value should be set to 1 to prevent the IGMP traffic from leaving the host’s network. The second required function is for hosts that are connected to more than one network. The host should only transmit multicast traffic on one of the directly connected networks because, in the multicasting paradigm, routers have the responsibility of forwarding multicast traffic to other networks. The third and last function specifies what a host should do when sending a multicast packet to a group of which the host is also a member. The transmitted multicast packet should be looped back to the host and the received packet that the host just sent should be discarded.