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PIM and IGMP

A router drops the multicast traffic, unless a multicast routing protocol is configured. Multicast routing protocols ensure that data sent from a multicast source are received only by the desired multicast clients. A group of multicast routing protocols designed for specific network environments is known as Protocol Independent Multicast or PIM. PIM can determine the path between the source and receiver using a particular unicast routing protocol. PIM must be configured at every interface of the participating router. Every router must also have the location of the Rendezvous Point manually configured. Then, unidirectional shared trees rooted at the Rendezvous Point will automatically be built for multicast distribution. Traffic from multicast sources reaches Rendezvous Point, and receivers pull it from a shared tree using Internet Group Management Protocol.

Multicast receivers talk IGMP to their local router, so, besides having PIM configured in every router, IGMP must also be configured in router where there could be a multicast receiver locally connected.

There are some PIM operating modes which are:

• PIM Dense Mode (PIM-DM)
• PIM Sparse Mode (PIM-SM)

The key difference between PIM Dense and Sparse Mode is the way, multicast tree is created. In PIM Dense Mode, all networks are flooded with the multicast traffic from the source and later, networks that don’t need the multicast are removed from the tree. “Root” of the multicast network is the network that contains the multicast source.

With PIM Sparse Mode, only networks that contain “requesting” multicast hosts are added to the multicast tree. A centralized PIM router, called the Rendezvous Point (RP), is elected to be the “root” router of the multicast tree. PIM routers build their tree towards the RP that are operating in Sparse Mode, instead towards the multicast source. To utilize the same multicast tree, the Rendezvous Point allows multiple multicast sources.

The above mentioned features are offered and supported by VCL-MX-50xx family of IP/MPLS Routers.