I finished up the rest of chapter 9 on Multicast by learning a little mor PIM theory along with simple configuration of multicast. When configuring PIM-DM (Dense Mode), it initially floods unicast traffic being sent by the source throughout the entire network. As each router receives multicast traffic via its RPF interface (the interface in the direction of the source), it forwards the multicast traffic to all of its PIM-DM neighbors.
PIM-DM prune messages are sent to stop unwanted traffic. Prune messages are sent on a RPF interface when the router has no downstream receivers for multicast traffic for that source. Prune messages are sent to non-RPF interfaces to shut off the flow of multicast traffic because it is arriving via an interface that is not the shortest path to the source.
PIM-SM (Sparse Mode) uses shared distribution trees with RP's (Rendezvous Points) but may uses source distribution trees as well. PIM-SM is based on a pull model so that traffic is forwarded only to those parts of the network that need it. PIM-SM uses an RP to coordinate forwarding of multicast traffic from a source to the receivers. PIM-SM is appropriate for wide-scale deployment for both densely and sparsly populated groups in the enterprise network. It is preferred over PIM-DM for all production networks regardless of size and membership density.
There are many optimizations and enhancements to PIM, including the following:
- Bidirectional PIM mode, which is designed for many-to-many applications (that is, many host all multicasting to each other)
- Source Specific Multicast (SSM), which is a variant of PIM-SM that builds only source specific shortest path trees and does not need an active RP for source-specific groups (in the address range 126.96.36.199/8)