Explaining PIM-Sparse Mode in Multicast and its different terminologies.

As we learn in the last blog Multicasts with PIM DM (Protocol Independent Dense Mode) in this blog we will learn why we need Sparse Mode in PIM. To understand this, we require PIM Dense mode disadvantages.

One of the big disadvantages of Multicast DM is, multicast stream sends (Flood) to all PIM enables routers whether they have required or not. To control this flooding PIM SM has come, in this blog, we will cover the below-mentioned topic-

  • PIM SM Terminology
  • Rendezvous Point (RP)
  • PIM SM Packet flow
  • Conclusion

PIM SM Terminology

PIM Sparse mode uses a shared tree to send the multicast traffic, it uses reverse path forwarding to create the path. It is very frequently used in current industries. The reason is, it controls flooding and unnecessary traffic utilization of the link. How it is working? To know this, we must know about Rendezvous Point (RP). Before learning RP, we will know the message types and another mechanism that is used in PIM Sparse mode.

Message types

Hello Message- Hello message like PIM DM, is used to establish the PIM neighbor-ship between PIM enable routers.

Join/Prune Message- Join message used by the router has required any multicast stream, they will send join message to RP only. Prune message only use to if no router joined to receive the multicast stream.

Register Message- So basically register message use between the initiator of multicast stream router to RP (Rendezvous Point) router.

Register stop message- Resister stop message use when RP router received the multicast traffic as RP acknowledged with register stop message.

Rendezvous Point (RP)

Rendezvous Point (RP) is use in PIM Sparse Mode. RP is the main reason to prevent the flooding of the stream, it is like a meeting point and all PIM routers meet on RP to join many multicast groups. There are two types to configure the RP-

Static (Manual) Rendezvous Point (RP)- If we are going to configure static RP, we need to configure one by one each router. Because we need to share the RP inform all routers. What will happen if any organization has a lot of routers which are working on PIM SM, in this scenario we need to configure one by one on all routers of RP information which is not an easy task. In this scenario, we need Dynamic Rendezvous Point.

Dynamic Rendezvous Point (RP)- To configure the Dynamic RP we need any protocol, there are several protocols used to configure the Dynamic RP.

  • PIM BSR (Bootstrap Router)- PIM BSR is not a Cisco propriety protocol, it is an open standard. It only works on PIM version-2.
  • PIM Auto-RP (Auto Rendezvous Point)- Auto-RP is the cisco propriety protocol that is only used in cisco devices. It works on both PIM version-1 and Version-2.

PIM SM Packet flow

From the packet flow perspective, the Video stream server will send the stream to their router which has PIM enabled.

Again, in all PIM-enabled routers, we will make (configure) one or more RP (R5) routers that are responsible to send a join message to the router (R1) which has initiated the multicast stream.

After the join message, RP will get the all-multicast stream and after that, if any user IGMP (Internet Group Management Protocol) reports for a multicast stream, on that time RP router will provide the same multicast group stream.

To sum up we can say PIM Sparse mode resolves the flooding of multicast on all PIM neighbors routers. The main role of this prevention is RP(Rendezvous Point). PIM sparse mode creates a shared tree and uses reverse path forwarding to share the multicast stream. For more configuration and implementation perspectives kindly reach us through our website or call on  +91 9773973971.

Brijesh Yadav
Network Consultant Engineer


Team ZT

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