Introduction to Multicast

This topic is to discuss the following lesson:

https://networklessons.com/multicast/introduction-to-multicast/

Very clear explanation Rene.

Davis

Solid information as usual. Thanks for the breakdown. Keep’em coming sir!

Awesome Rene!!!
You are the only one who can teach in a very clear way.
God Bless you :slight_smile:

Hi ,
nice explanation . But I have a doubt .
multicast is better in terms of saving bandwidth in comparision to unicast . Then why do data streaming sites like youtube , netflix etc donot use multicast instead of using a vast amount of unicast addresses .
Is the lesser amount of multicast group addresses in Class D the reason ? Kindly explain this to me .

Hello Ashutosh

First of all, services like Netflix and Youtube are not streaming services. They are video on demand services. This means that they implicitly require unicast functionality. The video that you watch on Youtube is being watched only by you at that specific time and no one else. Someone else may click on it a few seconds before or after you, but this is a uniquely separate event.

Streaming services that are candidates for using multicast include online radio and the broadcasting of live events over the Internet. However there are two reasons why they are not implemented:

  1. ISPs in general filter PIM and IGMP packets that are necessary to make multicast work
  2. When expanding to large scale multicast, the overhead required outweighs the benefits of the service

Concerning the first, multicast requires a multicast routing protocol such as Protocol Independent Multicast (PIM) which is the most popular one, although there are others. In order for this protocol to function on a network, it must be available end to end. PIM doesn’t normally run between ISP boundaries as it would be pretty complicated to do so. The protocol itself wasn’t designed for such a scale.

Secondly, keep in mind that multicast implicitly requires that end user device actions impact the forwarding state of a multicast-enabled IP router on the Internet. This means that for any multicast event, all end users (which may be from thousands of devices to millions) must have some way to inform multicast routers on the Internet of how the multicast routing should take place. When we are talking about such a large scale, the huge overhead required negates the advantages that multicast would give. This is why unicast is preferred at such scales.

In general, multicast is a technology that is used within a private network. It has been designed for such scales and thus is not implemented over the Internet. Having said that, some ISPs do offer multicast services WITHIN THEIR NETWORKS for enterprise customers, but those are special case scenarios.

I hope this has been informative and helpful!

Laz

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Hi Rene ,

Just got a one doubt after reading this , can you please guide ?
1.Is igmp a L3/L2 protocol ?
2.Does multicast router(end router which has to forward multicast traffic to host ) maintains any kind of exclusive table to map entries for interested hosts in downstream or igmp snooping enabled switch uses for this purpose ?

Regards,
Sameer

Hello Sameer

IGMP is a L3 protocol. It functions at L3 similarly to other network management protocols such as ICMP.

IGMP enabled routers will maintain a multicast group where a multicast address will be associated with the IP addresses of the hosts that have requested to be part of the multicast group. The show ip igmp groups command will show the IP addresses of the hosts that are associated with the specified multicast IP address.

On the other hand, IGMP snooping enabled switches will not maintain any such IP address associations (unless it is a L3 switch that performs IGMP functions of a multicast router). IGMP snooping is an exclusively L2 feature that cooperates with the functionality of multicast. It associates multicast MAC addresses with specific ports, those ports where hosts that are part of specific multicast groups reside.

More about IGMP Snooping can be found here.


Also, it is important to note that IGMP snooping must operate in conjunction with a multicast router. IGMP snooping on its own will not operate correctly. For more on this, take a look at this lesson:

I hope this has been helpful!

Laz

Very Good Write Up dear …/Thx

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By far, your explanation of the multicast components in play are the most digestible I’ve come across during my CCNA prep.

New to the Network Lessons Community (joined 6.4.19) and I’m so excited to leverage this resource for my CCNA Routing and Switching exam prep.

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Hi LAz ,

So I can say that by IGMP snooping the switch maintains the CAM table , or something like that ?

and to support IGMP snooping features we have deploy any special L2 switch? … I belief traditional CAT-Switches do not have such type of intelligence…

Thanks.

Hello Tanmoy

Yes, IGMP snooping is a feature which uses the CAM table of the switch to keep track of specific multicast MAC addresses and their correspondence to particular interfaces such that any traffic destined for these MAC addresses will not be broadcast to all ports, but only to those particular ports.

IGMP is a mature protocol that has been around for a long time. It is generally available in both L2 and L3 switches. According to this Cisco command reference, the command has been around since IOS version 12.0(5.2)WC(1) which if I’m not mistaken came out some time in 2001. You are correct however that some CatOS switches didn’t support it. You can see the compatibility matrix for IGMP snooping for Cisco devices at the following link:

I hope this has been helpful!

Laz

Hi, I have two questions regarding IGMP. So when does the switch forward multicast traffic ? second question would be , In the document you have mentioned that switch uses IGMP snooping ( hears from router for the multicast groups ). Does this mean always when a switch wants to forward multicast traffic an router should be there for the switch to tell about multicast groups ( destination ) ?

Hello Dakshinamurthy

If IGMP snooping is disabled, a switch will treat multicast traffic in the same way it treats broadcast traffic. It will always forward multicast traffic out of every port on the switch (or out of every port on the same VLAN, if multiple VLANs are configured) except the one it received the traffic on. If IGMP snooping is enabled, a switch will only forward multicast traffic out of the ports on which there are listeners to the specific multicast group IP address.

This is an excellent observation. When a multicast router exists on the network, IGMP snooping keeps track of the port through which the router is reachable, and uses that in its IGMP snooping mechanism. However, you may have a topology where there is no router, where you have several switches connected together in a single network segment, and you have a multicast source with several multicast receivers. Even without a router, it is possible to set up IGMP snooping. You can find out detailed information about how to do this at the following lesson:

I hope this has been helpful!

Laz

Thank you so much Lagapides. That explains it.