Network Kings

What is Multicast Routing Protocols? – Explained

multicast routing protocol

Before we dwell deep into multicast routing protocol, let us discuss the modes of transmission.

Transmission of the packet can be done in three ways, namely-

  • Unicast

In the case of unicast, the packet is sent from one sender to one receiver, i.e., it is one-to-one communication.

  • Broadcast

In the case of broadcast, the message is sent from one sender to all the connected and possible receivers, i.e., it is one to all communication.

  • Multicast

In the case of multicast, the message is sent from one sender to all the receivers who are interested to receive that particular traffic, i.e., it is one-to-many communication. 


So generally, when a router receives a packet, it looks for the destination address, finds the best path of the destination by looking into the routing table, and then transmits the packet using the best path. This is a case of unicast routing, but what happens when a router receives a multicast packet? What happens when it receives a packet with a multicast address?

In this article, we will be discussing multicast routing protocol in brief. 

What is Multicast Routing Protocol?

Multicasting is a form of group communication where a sender simultaneously sends data to several receivers or network nodes. One or more senders can send data packets to numerous receivers simultaneously over LANs or WANs using multicasting, which is a type of one-to-many and many-to-many communication.

Multicasting is particularly useful for applications such as streaming video, online gaming, content distribution, and various forms of group communication.

NOTE: Since multiple nodes can receive data at once through this process, network congestion and bandwidth utilization reduce.

Where is Multicast Routing used?

Multicast routing is used in the following technologies: – 

  • Video on demand 
  • Voice over IP 
  • Video streaming 
  • IP television (IPTV) 

When using multicast routing, the sender creates a multicast group and sends data packets to a particular multicast IP address. The network’s routers then use multicast routing protocols to choose the most efficient route for transmitting the packets to each recipient who has subscribed to the multicast group.

Which Multicast Routing Protocols are used?

In the case of multicast routing, the following multicast routing protocol is used: –  

  • Protocol Independent Multicast (PIM) 
  • Distance Vector Multicast Routing Protocol (DVMRP): –  
  • Multicast Open Shortest Path First (MOSPF) 

What is Protocol Independent Multicast (PIM)?

PIM forwards multicast packets to recipients using a shared or source-specific tree.

A family of multicast routing systems called PIM is made to operate with any unicast routing mechanism. PIM allows routers to dynamically create multicast distribution trees, which establish the route for multicast traffic to be forwarded from a sender to numerous receivers.

What are the communication kinds of PIM?

There have two modes of communication in PIM. 

  • PIM Dense Mode (PIM-DM)

It is designed for the network where there are many receivers interested in a multicast group. In dense Mode, it is assumed that most devices in the network are interested to receive the multicast packet hence, the router floods the multicast packet to ensure that it can reach every part of the network and then prunes it back to avoid sending data to a device that is not interested in receiving the packet. 

The host sends a prune message to the router to tell it that they do not need this multicast packet. PIM-DM is less commonly used as compared to PIM-SM due to its flooding behavior, which can cause unnecessary traffic in large networks. 

  • PIM Sparse Mode (PIM-SM)

It is designed for networks where multicast traffic is sparse, meaning that there are only a few receivers for a particular multicast group. This mode is used when only a few subnets of devices are interested in receiving the multicast data. Since not all connected devices want to receive the multicast data, routers do not flood the multicast packet in the network. 

In Sparse Mode, the host signals the interest in receiving the multicast packet by sending the join message and hence router is capable to identify the host which is interested in receiving this multicast packet and sending it only to the active joiners. SM then builds a multicast distribution tree. It is used in the case of large networks where multicast data does not need to reach every connected device. This mode is more efficient than Dense Mode. 


NOTE: It is important to note that in practice, multicast routing uses a combination of both modes to make a balance between efficiency and coverage. 

DVMRP is an older multicast routing system that determines multicast forwarding paths using a distance vector technique. While MOSPF is an enhancement of the OSPF unicast routing system that builds multicast distribution trees using the same link-state information.

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