What Is IGMP? Internet Group Management Protocol Explained

Introduction 

IGMP helps IPv4 networks keep track of devices that want to receive multicast traffic. Multicast is used to send a live stream to multiple computers in the network without needing to have a stream sent to each individual computer.

But how does a router know which connected networks have interested receivers? That’s where IGMP comes in. In this blog, we will discuss what IGMP is actually responsible for, how its messages work, and its different versions.

Let’s start by first understanding what exactly IGMP is.

What Is IGMP?

The Internet Group Management Protocol (IGMP) is a network layer protocol that manages IPv4 multicast group membership. Hosts use it to tell nearby multicast routers which multicast groups they want to receive traffic from.

IGMP carries membership information. The application data, such as a video stream, travels separately. IGMP runs directly over IPv4 using IP protocol number 2, without TCP or UDP ports. IPv6 uses Multicast Listener Discovery (MLD) for the corresponding membership function.

Example: Monitoring a Factory Inspection Camera

Imagine a factory camera sending an 8 Mbps inspection feed to multicast group 239.10.20.30. The camera is on VLAN 10. Three monitoring computers are on VLAN 20, connected through a multicast-enabled router.

1. Camera: 10.10.10.50

1. Monitoring computers: 10.20.20.11, 10.20.20.12 and 10.20.20.13

1. Group carrying the feed: 239.10.20.30

When an engineer opens the monitoring application, the computer joins that multicast group and sends an IGMP membership report. The router learns that VLAN 20 has an interested receiver. With multicast forwarding configured, it delivers the camera feed into that VLAN.

If all three computers join, the router can send one 8 Mbps stream onto their shared VLAN connection. The access switch then replicates it to the required receiver ports. This avoids sending three separate 8 Mbps streams across that shared connection, excluding protocol overhead.

The computers keep their individual IP addresses. The multicast address identifies the group, not a replacement address for each computer.

Why Do Networks Need IGMP?

A multicast source sends packets to a group address. It does not need a list of every receiver.

However, the router serving the receivers needs to know whether anyone on a connected network wants that group’s traffic. IGMP supplies this information and refreshes it as membership changes.

The router generally tracks whether a group has interested receivers on an interface, rather than maintaining a complete list of individual viewers. That distinction matters: one computer leaving does not mean the whole VLAN should lose the stream.

How IGMP Works?

IGMP works through messages exchanged between hosts and local multicast routers. These messages tell routers which multicast groups have interested receivers and whether those receivers still want the traffic.

1. The Host Reports Its Interest

When an application needs multicast traffic, it asks the operating system to join the required group. The host can send a membership report without waiting for a router to send a membership query first.

This report tells local multicast routers that a receiver wants traffic for that group. Joining is communicated through membership reports; there is no separate message type called “Join.”

2. The Router Maintains Membership Information

The router records which multicast groups have interested receivers on each connected network. It also maintains timers to keep this information current.

If the multicast source is across other routers, a routing protocol such as PIM can establish the forwarding path. IGMP provides the local membership information that helps determine where traffic is needed.

3. A Querier Checks for Active Receivers 

An IGMP querier periodically sends membership queries. Hosts that still want multicast traffic respond with reports, allowing routers to refresh their membership information.

Normally, one elected querier performs this role on each subnet. In IGMPv2 and IGMPv3, the eligible router with the lowest IPv4 address becomes the querier.

Response timers spread out replies. IGMPv1 and IGMPv2 also reduce duplicate reports: a host can cancel its pending response after hearing another member report the same group.

4. Membership Updates When Receivers Leave

The process for leaving depends on the IGMP version. IGMPv1 relies on membership timers expiring when reports stop arriving.

With IGMPv2, a departing host can send a Leave Group message. The querier checks whether other receivers remain before the router stops forwarding that group’s traffic onto the network.

IGMPv3 uses membership reports to communicate changes in reception requirements, including source preferences. Traffic continues wherever interested receivers remain.

Types of IGMP Messages

IGMP message types describe what a message does. They are different from protocol versions.

Message	Sent by	Purpose
Membership Query	Querier	Checks receiver interest
Membership Report	Receiver	Announces or refreshes reception interest
Leave Group	IGMPv2 receiver	Signals that it is leaving a group

IGMP queries become more specific across protocol versions:

1. General Query: Checks membership across all groups; available in all three versions.

1. Group-Specific Query: Checks one group; supported by IGMPv2 and IGMPv3.

1. Group-and-Source-Specific Query: Checks interest in particular sources for a group; supported by IGMPv3.

IGMPv3 uses report records to communicate membership changes instead of introducing a separate v3 Leave Group message.

IGMP Versions: v1 vs v2 vs v3

Each version adds more control over membership management.

Version	Main capability	Leaving or filtering behaviour
IGMPv1	Basic queries and reports	No explicit leave message; membership expires
IGMPv2	Faster detection of departures	Adds Leave Group and group-specific queries
IGMPv3	Source filtering	Reports which sources a receiver wants or excludes

IGMPv1 established the basic query-and-report process. IGMPv2 improved how quickly routers detect that the last receiver has left. IGMPv3 added source selection, which supports source-specific multicast (SSM).

What Does IGMPv3 Source Filtering Mean?

IGMPv3 source filtering allows receivers to specify which sources they want traffic from within a multicast group. It supports two reception modes:

1. INCLUDE: Receive traffic only from the listed sources.

1. EXCLUDE: Receive traffic from all sources except those listed.

This gives receivers control over both the multicast group and the senders whose traffic they receive.

Source-specific multicast (SSM) uses this capability to identify a stream by its source and group address, written as (S, G). The IPv4 range 232.0.0.0/8 is reserved for SSM.

What Is IGMP Snooping?

IGMP snooping is a switch feature that observes membership messages and learns which ports need multicast traffic. In the factory example, a switch can forward the camera stream to the monitoring computers ports while keeping it off unrelated receiver ports. It also accounts for ports connected to multicast routers.

Without snooping or another multicast filtering mechanism, a switch may flood multicast traffic across the VLAN. Snooping reduces that unnecessary delivery. A snooping switch also needs membership information to stay current. Check that the VLAN has an active querier; some switches provide a snooping querier function when no multicast router is present.

IGMP vs PIM: What Is the Difference?

IGMP manages receiver membership on a local network. PIM helps routers build multicast forwarding paths between networks.

For a camera and viewer separated by several routers, IGMP reports the viewer’s interest at the receiving end. PIM uses that interest to help establish delivery across the routed network. IGMP alone does not build the entire path.

Common IGMP Problems to Check

If a multicast feed is missing, check these parts of the setup:

1. Group membership: Has the application joined the correct group on the correct network interface?

1. Querier activity: Are queries and responses keeping membership entries alive?

1. Switch forwarding: Does the snooping table include the receiver port and appropriate router ports?

1. Routed delivery: Is multicast forwarding configured between the source and receiver VLANs?

1. Version support: Does a source-specific application have the required IGMPv3 support?

Treat these as separate checks. A valid membership report shows receiver interest, but successful delivery also depends on the forwarding path.

Conclusion

IGMP enables IPv4 hosts and multicast routers to manage multicast group membership efficiently. IGMPv1, IGMPv2 and IGMPv3 progressively add capabilities such as faster leave detection and source filtering. In real networks, IGMP works alongside IGMP snooping and multicast routing protocols such as PIM to control where multicast traffic is delivered. Understanding these components helps network engineers troubleshoot multicast applications and design more efficient IPv4 networks.

Frequently Asked Questions

Q1. What is IGMP Used For?

IGMP is used to manage IPv4 multicast group membership. It allows hosts to indicate which multicast groups they want to receive traffic from, while multicast routers use this information to maintain receiver membership.

Q2. Why Should You Enable IGMP Snooping?

IGMP snooping helps switches forward multicast traffic only to ports that have interested receivers, reducing unnecessary multicast flooding within a VLAN. An active IGMP querier is also needed to keep membership information current.

Q3 What is the Difference Between IGMP and ICMP Protocol?

ICMP is used for reporting network errors and for diagnostic purposes, and IGMP is used for managing joining a multicast group between hosts running on the referenced network and network multicast routers in the vicinity.

Q4 Does IGMP Use TCP or UDP?

IGMP uses neither TCP nor UDP. It runs directly over IPv4 using IP protocol number 2, without transport layer ports.