Configuring IP Multicast
What is Protocol Independent Multicast – PIM?

Protocol Independent Multicast (PIM) is a multicast routing protocol that makes packet-forwarding decisions independent of standard or unicast IP routing protocols such as EIGRP. At the same time, PIM can leverage unicast routing tables to perform multicast forwarding functions. For example, PIM uses the unicast routing table to perform the RPF check function. Unlike unicast routing protocols, PIM does not send and receive routing updates between routers.

PIM has three forwarding modes:

  • PIM Dense Mode (PIM-DM) – This mode uses a push model to flood multicast traffic to every corner of the network. Routers located throughout the tree can then prune the flow if they receive no requests for the particular multicast group flow. This method would be efficient in certain deployments in which there are typically active receivers on every subnet in the network.
  • PIM Sparse Mode (PIM-SM) – This mode uses a pull model to deliver multicast traffic. Only network segments with active receivers explicitly requesting the flow will receive the traffic for a multicast group. PIM-SM distributes information about active sources by forwarding data packets on the shared tree. Because PIM-SM uses shared trees, the source forwards its flow to a rendezvous point (RP), which then sends it to any requesting routers.
  • PIM Sparse-dense Mode – This mode allows individual groups to be run in either sparse or dense mode depending on whether RP information is available for that group. If the router gleans RP information for a particular group, it will be treated as sparse mode; otherwise that group will be treated as dense mode.
NOTE:

Cisco strongly recommends sparse-dense mode.

PIM Version 1 and 2
PIM version 1 was Cisco proprietary. In addition to being an IEEE standard, Version 2 includes the following improvements:

  • A single, active rendezvous point (RP) exists per multicast group, with multiple backup RPs. This compares to multiple active RPs for the same group in PIM Version 1.
  • A bootstrap router (BSR) provides a fault-tolerant, automated RP discovery and distribution mechanism. Thus, routers dynamically learn the group-to-RP mappings.
  • Sparse mode and dense mode are properties of a group, as opposed to an interface. PIM Join and Prune messages have more flexible encodings for multiple address families.
  • Register messages to an RP indicate whether they were sent by a border router or by a designated router.
  • PIM packets are no longer inside IGMP packets; they are stand-alone packets.

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