IGMP Version 3

IGMP version 3 adds support for “source filtering”. IGMP version 1 and version 2 allow hosts to join multicast groups but they don’t check the source of the traffic. Any source is able to receive traffic to the multicast group(s) that they joined.

With source filtering, we can join multicast groups but only from specified source addresses. IGMP version 3 is a requirement for SSM (Source Specific Multicast) which we will cover in another lesson.

Why is this useful? Let me give you an example:

Multicast video server four hosts

Above we have a video server that is streaming multicast traffic on the network using destination address 239.1.1.1. There are four hosts listening to this traffic, life is good. Suddenly something happens:

multicast attacker sending traffic

An attacker didn’t like the video stream and decided to stream his favorite video to destination address 239.1.1.1.1. Since we don’t check the source address, everyone will receive the traffic from our attacker. It’s also possible to send bogus traffic and create a DoS attack like this.

IGMP version 1 and 2 don’t have any protection against this.

With IGMP version 3, our hosts can be configured to receive multicast traffic only from specified source addresses. Let’s see how this works, I’ll use the following topology for this:

Multicast IGMP Version 3 topology

We will only use two devices, one multicast enabled router and a host device. I’m using a Cisco router as the host device as well.

Let’s start with R1:

R1(config)#ip multicast-routing
R1(config)#interface GigabitEthernet 0/1
R1(config-if)#ip pim sparse-mode 
R1(config-if)#ip igmp version 3

Our router requires multicast routing and PIM should be enabled on the interface. The default version of IGMP is 2 so we’ll change it to version 3. Before we let H1 join a multicast group, let’s enable debugging on both devices:

R1 & H1#debug ip igmp 
IGMP debugging is on

R1 will start sending membership general queries like the one below:

multicast igmp version 3 membership query general

Let’s configure H1 to join a multicast group:

H1(config)#interface GigabitEthernet 0/1
H1(config-if)#ip igmp join-group 239.1.1.1 ?
  source  Include SSM source
  <cr>

Besides configuring a group, I can configure the host to include a source address. Let’s pick something:

H1(config-if)#ip igmp join-group 239.1.1.1 1.1.1.1

H1 will now include the source address in its membership report messages. Here’s what you will see on the console:

H1#
IGMP(0): WAVL Insert group: 239.1.1.1 interface: GigabitEthernet0/1Successful
IGMP(0): Create source 1.1.1.1
IGMP(0): Building v3 Report on GigabitEthernet0/1
IGMP(0): Add Group Record for 239.1.1.1, type 5
IGMP(0): Add Source Record 1.1.1.1
IGMP(0): Add Group Record for 239.1.1.1, type 6
IGMP(0): No sources to add, group record removed from report
IGMP(0): Send unsolicited v3 Report with 1 group records on GigabitEthernet0/1
IGMP(0): Building v3 Report on GigabitEthernet0/1
IGMP(0): Add Group Record for 239.1.1.1, type 5
IGMP(0): Add Source Record 1.1.1.1
IGMP(0): Add Group Record for 239.1.1.1, type 6
IGMP(0): No sources to add, group record removed from report
IGMP(0): Send unsolicited v3 Report with 1 group records on GigabitEthernet0/1

H1 sends two membership report messages. The first message includes the multicast group address and source address that we want to receive. The second message includes the “mode”. There are two modes:

  • Include: this is a list of source addresses that we accept multicast traffic from, everything else should not be forwarded.
  • Exclude: this is a list of source addresses that we refuse to accept multicast traffic from, everything else should be forwarded.

Here’s what it looks like in wireshark:

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Forum Replies

  1. Hello Rene,
    A few questions and confusions.

    1. When a multicast address is being used as a group address(for example 239.1.1.1), this address is assigned to the multicast server and this address has to have route in the network design because whenever a host will encapsulate an IP packet, it will use its own address as the source and 239.1.1.1 address as the destination address. Is this correct?
    2. Would you also please give me a real life scenarion where multicast is used?
    3. In your IGMP version 3 example, 1.1.1.1 is being used as the source address. What is this add
    ... Continue reading in our forum

  2. Hello Azm

    Well, not quite. The multicast address is not assigned to the multicast server,

    ... Continue reading in our forum

  3. Hi Azm,

    Multicast isn’t really used on the Internet. It would make a lot of sense to use multicast for online radio but in reality, it’s unicasted everywhere. There is no “global” multicast network that spans multiple ISPs. One application that was common back in the days on the LAN was Norton Ghost to send a system image to all computers. Without multicast, you had to unicast huge images to all computers which wasn’t very efficient.

    When your computer wants to receive a multicast stream, it uses IGMP to “report” which multicast group it wants to receive. IGMP

    ... Continue reading in our forum

  4. Hi Rene,

    What is the purpose of sending two membership report from H1, one determines there is a new address and the other determines the mode, I mean why not be in one membership report ???

  5. Hi Hussein,

    To be honest, I have no idea…I guess this is one of those “that’s how they designed it” answers. They probably could have been combined in a single packet but for some reason, they decided to go for two packets.

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