IGMP Version 1

Lessons Discussion
1

This topic is to discuss the following lesson:

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2

Rene,

Great lesson, however I have a question about for the host don’t we need ip pim sparse-mode? I can see on the router we have ip pim sparse-mode.

Please clarify.

Hamood

 

 

3

Hi Hamood,

PIM is only required on multicast routers. I’m using routers as hosts but the only thing they need is “igmp join” to simulate a host. There’s no need for multicast routing or PIM on these host routers.

Rene

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4

great post.

with respect to below 2 statements: what is the diff between multicast-routing and PIM command ? both seems to be doing same thing - process the IGMP traffic.

1.First we enabled multicast routing globally, this is required for the router to process IGMP traffic.
2.We enabled PIM on the interface. PIM is used for multicast routing between routers and is also required for the router to process IGMP traffic.

Also
a. from where .40 address came ? does router generates it randomly ?
b. group address 239.1.1.1 who decides and gives this IP in real time environment ?

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5

Abhishek,
Think of it as acting the same like a regular dynamic routing protocol. So, for example, you can enable EIGRP to run on a particular interface, but it won’t do anything unless you also turn on EIGRP to run globally. Likewise, you have to tell the router that you want it to route multicast traffic globally, but you also need to tell it which interfaces need to be listening for the multicast traffic.

6

Thx Andrew, got it now .

7

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.

8

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.

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9

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?

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10

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

11

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

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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

13

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

14

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

15

Hi Rene,

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

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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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17

Thanks Rene your answer is helpful for me

18

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

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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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20

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.