IGMP Version 1

Rene,

In your conclusion you mention “one of the downsides of IGMP version 1 is that the router will keep forwarding multicast traffic even when there are no more hosts interested in our traffic.”

Do you mean it will continue to send membership queries even though no host care to listen?
I am just trying to understand what “traffic” it will keep sending that makes version somewhat inefficient, thanks.

IGMP version 1 only had two messages: the membership query and membership reply. There was no message that allowed a host to inform that he didn’t want to receive multicast traffic for a certain group anymore. It was possible that a router would forward packets to a certain multicast group with no listeners anymore…

So with traffic, I meant the actual video/audio/application traffic for the multicast group that the listeners are interested in.

Rene, Good network article.

Need some clarification.
What are the IPs 224.0.0.1 and 239.1.1.1. Are these IPs default for Membership Report and Query process. Why the host need to join to 239.1.1.1 group, Is it the default group or global group for multicast? Can there be other group IPs can be configured?

Hello Pavithra

Both of these addresses are multicast addresses, however each one is used for a different purpose. The 224.0.0.1 IP address is a well known multicast IP and it addresses all the hosts on a network segment. This is used when the router sends out the membership query because all hosts are asked if they are (still) interested in receiving the multicast traffic.

The 239.1.1.1 mutlicast IP address refers to a multicast group only. This is the IP address that corresponds ONLY to those hosts that have expressed interest in taking part in the specific group. This is the actual IP address which is used to send the multicast traffic to the hosts participating in the group.

I hope this has been helpful!

Laz

Hi,
Let’s assume that there is no R1 , only two hosts connected to a switch in same vlan ,
What will happen in that case ?
Thanks

Hi,

R1#show ip igmp groups 239.1.1.1
IGMP Connected Group Membership
Group Address    Interface                Uptime    Expires   Last Reporter   Group Accounted
239.1.1.1        GigabitEthernet0/1       19:22:56  00:01:07  192.168.1.101

Can you explain
Why the host cannot see 192.168.1.102 cant see here
Thanks

Hello Sims

If there is no router, IGMP version 1 will not function correctly. This is because the router is the one that manages the multicast membership of each host.

I hope this has been helpful.

Laz

Hello again Sims

There are several reasons why this would be the case:

  1. The IGMP join-group command may not be configured on H2
  2. There may be an IGMP version mismatch
  3. Make sure there is connectivity between H2 and the router

Try using some debug commands described in the lesson to see if there is an attempt to include the host in the group but there is some kind of error. The debugs should help you out there.

I hope this has been helpful!

Laz

Hi Rene,

How can I verify IP multicast-outing is enable or not by using show commands ?

There are two options that come to mind:

R1#show ip multicast                   
        Multicast Routing: enabled
        Multicast Multipath: disabled
        Multicast Route limit: No limit
        Multicast Triggered RPF check: enabled
        Multicast Fallback group mode: Sparse
        Multicast DVMRP Interoperability: disabled

Or just check the running config:

R1#show run | include multicast-routing
ip multicast-routing
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Thanks Rene your answer is helpful for me

Hi Larazos, Rene,

With regards to the Administratively-scoped (local) multicast addresses, am i right to say that we could select from the ranges below when assigning the multicast group address?

239.0.0.0 - 239.255.255.255

What is the exact rule for assigning the IP address for the multicast group?
Can we also select from the below addresses?
224.0.0.0 - 224.0.0.255 - Reserved for special “well-known” multicast addresses
224.01.0 - 238.255.255.255 - Globally-scoped

Hello Recto

The 239.0.0.0 to 239.255.255.255 range of multicast IP addresses known as “Administratively scoped” addresses. These are defined by RFC 2365 and are reserved for private use within an organization. So to answer your question, yes, when configuring multicast groups it is best practice to use these addresses.

Having said that, as you probably know, there are various special-use multicast addresses that have various purposes as defined by the IANA and by various RFCs. Although I don’t like linking to Wikipedia, I find that their summary of the IPv4 multicast address groups is quite comprehensive and complete, and you can find it at the following link:

So can you use these other multicast addresses to define a multicast group? Well, yes, nothing stops you from doing that. However, if you overlap with various well known multicast addresses such as those used by OSPF or EIGRP, then this will result in the malfunctioning of any such routing protocols you may have running on your network.

Even though technically you can indeed configure them, practically, it can present a problem on your network.

I hope this has been helpful!

Laz

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Rene.
I enabled debug ip igmp on cisco routers via GNS3 but I can’t see the debug report.
what is your lab environment? is it real router or emulator?
I tried with IOSv image and c7200 router both but same result.
Thanks.

Hello Jiwon

Both real equipment as well as the GNS3 should have the same debug output. When implementing debugging, make sure that you also employ the terminal monitor command in order to tell the device that all debug output should appear on the terminal screen. Also, verify that IGMP is indeed enabled and that you have achieved the necessary connectivity between the hosts and R1 before implementing the commands on the hosts.

Take a look at these and let us know your results!

I hope this has been helpful!

Laz

Hi Laz, Can you explain it please ?Please correct me if am wrong. 224.0.0.1 addresses all the host on a network segment. This IP is used by the router to send membership query to all host to ask whoever interested in multicast traffic. And then followed by this you say that 239.1.1.1 multicast IP is the actual IP address that is used to send the multicast traffic to the host participating in the group ? I don’t understand here.
Do you mean 239.1.1.1 is the multicast IP address that host uses when they are sending the membership report ?
Please confirm

Hello Dakshinamurthy

Yes, this is correct. All IP hosts on a segment will receive such a message. This multicast address only remains within the segment, and is not routable to any other network segments.

This multicast IP address is the multicast group which the hosts in this lesson want to join. This is seen by the following commands issued on the interface of H1 and H2:

H1(config)#interface GigabitEthernet 0/1
H1(config-if)#ip igmp version 1
H1(config-if)#ip igmp join-group 239.1.1.1

Unlike the 224.0.0.1 address, which is a well known and reserved address, the 239.1.1.1 multicast group address is configurable. This means that some hosts choose to join it, while others may choose not to. It is the destination address used by multicast sources to send their content to. A video server, for example, would use this as the destination address to reach all hosts interested in its multicast stream.

Yes, this address is also used by hosts to send their membership reports. The membership reports actually tell the multicast router that the host sending the report wants to receive multicast traffic destined for this particular multicast IP address.

I hope this has been helpful!

Laz

Rene,

Speaking of enabling multicast routing, in your topology, H1 and H2 are Cisco routers, but in your configuration example, I did not see the command ip multicast-routing on H1/H2. For routers to participate in multicast, do you not have to enable on all routers in global config mode even when they are behaving as hosts??

Hello Ralph

As you correctly stated, H1 and H2 are actually routers, but are functioning as hosts. Within the framework of multicast, hosts (even if they are Cisco routers) do not perform multicast routing. So to answer your question, no, multicast routing does not (and actually should not) be enabled on routers that play the role of a host.

Think about it this way. If you have a Windows PC connected to the network, and it wants to participate in a multicast group, it can do so even through it does not have the capability of multicast routing itself. For hosts of any type to participate in multicast, they simply have to support IGMP and the functionality of joining of multicast groups.

I hope this has been helpful!

Laz

Yes…Thank you for the clarification. I’m using multicast more and more these days it seems!!

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