PIM Distribution Trees

A multicast router will create a distribution tree that will define the path multicast traffic follows to reach receiver devices.

There are two types of multicast distribution trees known as source trees. They are also known as shortest path trees, SPTs and shared trees.

Source Tree

A source tree is a multicast distribution tree where the source of the multicast stream is the root of the tree. There are branches that form a tree through the network to the receivers.

When the source tree has been built, it will utilise the shortest path from the source of the multicast traffic to the destination hosts. This is where the source tree gets the definition: shortest path tree.

The shortest path tree uses a forwarding state, known as (Source, Group), or (S, G).

The source is the sender of the multicast packets, or the source of the packet flows. The group is the multicast group address.

Since shortest path trees begin at the source of the multicast stream, every sending source requires to have a shortest path tree.

Shared Tree

A shared tree is a multicast distribution tree where the root is not at the source of the multicast stream. The root of source tree is defined at a router known as a rendezvous point, or RP.

Shared trees can also be known as RP trees (RPT) due to this characteristic.

Multicast traffic are forwarded down the shared tree according to the group address that the packets are addressed too, regardless of the source address.

The forwarding state of a RPT is known as (*, G)

Shared trees require fewer multicast entries. If more sources are introduced utilising the same multicast group, it will not require a separate entry in a shared tree as it would in a shortest path tree.

This advantage also works to the shared trees disadvantage. All receiver hosts in the shared tree will receive traffic sent by any host to the multicast group address. This can generate a lot of unwanted network traffic.

Without the policing of the source address in a shared tree, this can introduce a security issue – as any host can send data to the shared multicast address.