Introduction to Multicast

This topic is to discuss the following lesson:

Very clear explanation Rene.


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!



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 ?


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!


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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