Multicast - PIM Dense Mode and PIM Sparse Mode

In this section we will discuss about PIM, What is PIM?  What is PIM Dense mode and PIM Sparse mode?

Let’s start with Multicast PIM...

PIM – Protocol Independent Multicast, It a routing protocol for Multicast Traffic. As you enable Multicast with “ ip multicast-routing” in global configuration mode. The Router will be enabled for multicast. IGP routing protocols exchange network information of one router’s network with others. Same multicast Protocol will exchange multicast information of Routers with each other. 

                For better understanding, you can keep in mind. It works bit similar to IGP. Multicast protocols will share multicast groups information with each other.

PIM is a protocol which will enable multicast routing in Router. AS you enable Router interface for Multicast it will try to form neighbour ship on Multicast address 224.0.0.13.

                Multicast Routing is totally dependent on IGP routing. IGP routing will help multicast PIM to find best way to reach source. PIM will create a routing table which is known as mroute table (Multicast Routing table) which will consist multicast entries.

PIM is a collection of multicast routing protocol, Two major PIM protocols are -

PIM DENSE MODE and PIM SPARSE MODE

PIM DENSE MODE:

PIM Dense mode is more similar to Source tree. In dense mode Source Tree will be start from source of network who is generating multicast traffic and other Multicast host Routers will be acting as branches of Tree which are trying to access the multicast stream. In PIM dense mode Router will initially flood multicast packet across network. This process will help in creation of (S, G) entry in all routers. This process is repeated every three minutes in PIM dense mode.  In this process those routers which don’t need the multicast stream that router will prune back the traffic flow.

Check both the diagrams for better understanding....

                 

PIM Dense mode flooding example -

In this example, multicast traffic being sent by the source is flooded throughout the entire network.

As each router receives the multicast traffic via its RPF interface (the interface in the direction of the source), it forwards the multicast traffic to all of its PIM-Dense neighbors.

Note that this results in some traffic arriving via a non-RPF interface such as the case of the two routers in the center of the drawing.  (Packets arriving via the non-RPF interface are discarded.) These non-RPF flows are normal for the initial flooding of data and will be corrected by the normal PIM-DM pruning mechanism.

Some key points about PIM Dense Mode - 

• PIM Dense mode is also known as PUSH MODE.
• PIM dense is recommended for small networks.
• PIM dense mode is very easy to configure on Routers.
• PIM dense mode is not scalable.

For summary – PIM dense mode is flooded across the network, then Routers which don’t have request for that particular group will prune it. The PIM dense tree is formed. Same process is repeated every three minutes. This process is not efficient in large enterprises network. So PIM dense mode is used in small networks, to avoid more configurations and easy management.

Dense mode command:

Interface mode – ip pim dense mode.

PIM SPARSE MODE –

PIM Sparse mode similar to shared Tree. PIM sparse mode is having a central point of contact for all Multicast host routers. As the Routers will receive the request from host to join particular multicast group, They will check the multicast routing table which is received from PIM and in case of the router is not aware, The request will be forwarded to RP (Rendezvous Point). RP is a designated router in network selected by network administrator as per requirement/Topology. This router will be responsible for providing information about every multicast group to host. All multicast source will register with RP. RP will aware about all multicast group and how to reach to multicast source.

                As RP will receive request from host, It will update source information and path to host router. Same time host router will use IGP information and find the best path and reach to source.

                In sparse mode, RP plays very important role. We can configure RP in many ways. For Cisco routers we can configure Auto-RP, other way are Static RP in each router and BSR – Industry standard (bootstrap router). We will discuss about BSR in upcoming posts.

Now we need to understand. In sparse mode RP is playing major role. Here the model is like shared tree. You can designate a Central Router as RP and every Multicast host traffic will pass thru RP.

Go thru diagram for more understanding....

In sparse mode, every router will have (*,G) entry, As the multicast traffic will be active in router. Then it will add (S, G) entry.

Some key point of PIM Sparse Mode - 

• Sparse mode is recommended for structured Multicast network Design
• We can say, it is very useful in large networks. We can deploy redundant RP to manage our network efficiently.
• Sparse mode requires planning for deployment. It is having some more configurations as compared to dense mode.
• PIM sparse mode is pull mode.
• Sparse mode is scalable.

Sparse mode command –

Interface mode – ip pim sparse-mode

                                             I explained in very simple language, which will clear your concepts about PIM and PIM Dense mode and PIM Sparse mode easily.

Hope this information is useful to you..

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