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Multicast Basics

Hello folks Good day, Tried my level best to explain, Please do forgive if any mistakes and highlet the points if missed

Unicast :

In unicasting a piece of information is sent from one point to another point. Here there is one sender and one receiver.

Broadcast :

Broadcast describes a communication between one point to all other points. Here there will be one sender and the information is sent to all the connected receivers of a broadcast domain.


Multicasting describes where in a piece of information is sent from one or more points to a set of other points. In this case there will be one or more senders, and the information is distributed to a set of receivers. (Sometimes there may be no receivers or any other number of receivers)

Now this document is written with an intention to explain PIM and MSDP.

PIM (Protocol Independent Multicast)---->

PIM is a control plane protocol for multicast routing

PIM has two versions, Version 1 and Version 2. IOS by default uses Version 2.

PIM is used between the routers to figure how the multicast tree to be built from the sender down to the receiver or from the sender to Rendezvous point and from Rendezvous point to the receiver.

PIM is considered to be protocol independent as it doesn't advertise any topology information when it is building the multicast tree. It implies we need an IGP to make sure that the topology is loop free to begin with.

PIM uses loop prevention mechanism called RPF check but it doesn't advertise the topology information, it is the responsibility of IGP.


Example for how RPF is going to work

When sender Sends the multicast stream to R1,

R1 will tell it to R2 then

R2 will tell it R3 and R4 then

R3 will tell to R1 and R4 and

R4 will tell it R3 and R2, It means here there is a scope of looping, RPF's duty is to check, when the sender send a stream it will go in a reverse path and checks down the interface from which it is receiving the stream, if it can reach the sender via the interface it received the stream then it will allow the packet otherwise of a case it will drop the packet.

PIM has three modes for multicast tree to be built:

PIM Dense Mode

PIM Sparse Mode

PIM Sparse Dense Mode

PIM DENSE MODE: In PIM Dense mode (PIM-DM) is a multicast routing protocol that uses the

underlying Unicast routing information base to flood multicast

datagrams to all multicast routers. Prune messages are used to prevent future messages from propagating to routers without group membership information.

This is flood and prune method and is implicit joining way.

PIM SPARSE MODE : NO Traffic unless a receiver asks for it. It has two modes again they are Normal sparse mode (Group specific) and Source Specific Sparse Mode.

We need a RP when we are running Group Specific Mode and for Source Specific mode needs no RP.

PIM Sparse Dense Mode: PIM sparse Dense mode any group that is assigned with a RP will run in Sparse mode and any group that which is not assigned with RP will fall to Dense mode.

MSDP : MSDP (Multicast source Discovery Protocol) is used between multiple RPs in a sparse mode PIM network to signal each other to know who are the multicast senders in the network.

It is generally designed for Inter AS Multicast designs where there are two different Service Providers who needs to exchange the information of multicast sources.

We can also use it in Intra AS application in side same AS as anycast to find the nearest RP.

MSDP is not the replacement for PIM, in between the different AS we will still need to run the PIM for building the multicast tree. MSDP will only signals about multicast senders, it doesn't have anything to do with Receivers and building the tree.

Once we build the control plane for multicast then end host will signal with IGMP (Internet Group Management Protocol )saying I need to receive traffic from a specific group, then routers will signal each other with PIM saying these are senders and these are the receivers and they will deliver the Streaming.

PIM(with in the routers)

IGMP(used by hosts and adjacent routers)

"This is just a basic and still needs to study more about it" for more information we can browse to



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