The ASM mode is the default mode when you configure RPs.
Bidirectional shared trees (Bidir) is a PIM mode that, like the ASM mode, builds a shared tree between receivers and the RP but does not support switching over to a source tree when a new receiver is added to a group. In the Bidir mode, the router that is connected to a receiver is called the designated forwarder (DF) because multicast data can be forwarded directly from the designated router to the receiver without first going to the RP. The Bidir mode requires that you configure an RP.
The Bidir mode can reduce the amount of resources required on a router when there are many multicast sources and can continue to operate whether or not the RP is operational or connected.
Source-specific multicast (SSM) is a PIM mode that builds a source tree that originates at the designated router on the LAN segment that receives a request to join a multicast source. Source trees are built by sending PIM join messages in the direction of the source. The SSM mode does not require you to configure RPs.
The SSM mode allows receivers to connect to sources outside the PIM domain. PIM messages include the following:
Hello: The PIM process begins when the router establishes PIM neighbor adjacencies by sending PIM hello messages to the multicast address 224.0.0.13. Hello messages are sent periodically at an interval of 30 seconds. After all neighbors have replied, the PIM software chooses the router with the highest priority in each LAN segment as the designated router. The DR priority is based on a DR priority value in the PIM hello message. If the DR priority value is not supplied by all routers, or the priorities match, the highest IP address is used to elect the DR.
The hello message also contains a hold-time value, which is typically 3.5 times the hello interval. If this hold time expires without a subsequent hello message from its neighbor, the device detects a PIM failure on that link.
For security, you can configure an MD5 hash value that the PIM software uses to authenticate PIM hello messages with PIM neighbors.
Join-Prune: When the DR receives an IGMP membership report message from a receiver for a new group or source, the DR creates a tree to connect the receiver to the source by sending a PIM join message out the interface toward the rendezvous point (ASM or Bidir mode) or source (SSM mode). The rendezvous point is the root of a shared tree, which is used by all sources and hosts in the PIM domain in the ASM or the Bidir mode. SSM does not use an RP but builds a shortest path tree (SPT) that is the lowest cost path between the source and the receiver.
When the DR determines that the last host has left a group or source, it sends a PIM prune message to remove the path from the distribution tree.
The routers forward the join or prune action hop by hop up the multicast distribution tree to create (join) or tear down (prune) the path.
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