Multicast Introduction

Multicast Introduction

Multicast Introduction


Multicasting is a method through which a single source can able to send any data to selected multiple destination across layer-3 network.


  • You-Tube
  • Radio Programs
  • Corporate mgs to employee
  • TV Broadcasts

Basic Requirement of Multicast:

  • In order to run multicast traffic in any environment following requirement is required:
  • A dedicated Layer-3 address range for Multicast application, Multicast address must be used as destination address not source address
  • A specific multicast application used to generate multicast traffic
  • Following mechanism is used by host to inform device whether they want to receive multicast traffic or not.
    • IGMP -->Router
    • CGMP, IGMP Snooping -->Switch
  • For Multicast Switching and Routing following protocols are used.
  • Routing : PIM, MSDP
  • Switching: IGMP, CGMP, IGMP Snooping etc.

 How Multicast traffic is sent to Selected Users

Let’s understand the concept of Multicast by below diagram.

  • A Video Server is sending video streaming on multicast address
  • Host H1, H2 are not intended to receive but H3 and H4 are interested to receive this multicast video stream and they launched the application in PC to see that stream.
  • When Host H3 and H4 launch the application, these host joins the group and request to receive multicast video stream which is being sent to 255.5.5
  • H3 and H4 indicated this request to R2 about its desire.
  • Multicast application calculates the L2 multicast address from its L3 Multicast address and assign it to NIC of host H3 and H4 along with its BA Address.
  • Multicast Routing is already configured and is working on R1 and R2 and they will forward multicast traffic to destination switch over which H3 and H4 are connected.
  • Switch with the help of CGMP/IGMP snooping stores the Source and Destination information and on which port the multicast has to be sent
  • Now multicast traffic will flow from source to destination H3 and H4.

Multicast IP address:

Multicast IP address range is as per given rule:

  • Class D: -
  • In 32 bit address, the first four bit of first octet is reserved for bit 1110 = 224.
  • Last 28 bits are unstructured and can be used for address spacing.

Multicast Well known address used in today networks environment.


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