Bidirectional Shared Trees

Generally speaking, Bidirectional Shared Trees are multicast distribution trees where the branches of the tree are used for bidirectional traffic flow. That is, if router joins a bidirectional tree on behalf of a group member, that branch of the tree is not only used for the member to receive data, but if the member is also a sender, the same branch is used to send data.

The advantage of Bidirectional Shared Trees is that you can have many sources send on the same tree without the routers having to explicitly keep state for each source.

cisco has implemented Bi-directional Shared Tree PIM (Bidir-PIM) to reduce the amount of state routers have to keep. This helps to reduce memory, bandwidth, and CPU requirements for routers.

Bidir-PIM requires some enhancements to the PIM-SM protocol to support bidirectional shared-trees in addition to supporting unidirectional shared-trees, and unidirectional source-trees.

The cisco IOS implementation supports 3 modes for a group:

Dense-mode
Sparse-mode
Bidir-mode

That is, a single router could support all 3 modes for different groups at the same time. A router will treat a group in dense-mode if it does not know about an RP for the group. A router will treat a group in sparse-mode, if it knows
an RP for the group that hasn't advertised the group as being a bidir-mode group. Otherwise, the group is in Bidir-mode.

Bidir-mode has superset functionality over sparse-mode but doesn't create (S,G) state for the group. Explicit joins are used for members to join the shared tree however sources do not use Registers to get their data on the shared tree.

Debugging multicast connectivity is made simpler since routers only maintain a single (*,G) entry per group.

Configuring Bidir-PIM

Related documents

draft-farinacci-bidir-pim-01.txt

This draft describes the first protocol implemented by Cisco for Bidir-PIM. In this approach, uptree forwarding utilizes a new IP option called the Upstream Multicast Packet (UMP) option. The UMP option is inserted by the first-hop DR attached to a multicast source so the packet can travel natively hop-by-hop to the RP. If there are any members along the path from the source to the RP, they will get the packets at that point (opposed to getting them from the direction of the RP).

draft-kouvelas-pim-bidir-new-00.txt

This draft describes an improved method for Bidir-PIM which does not rely on the UMP option. This protocol was developed to significantly simplify forwarding procedures and to eliminate forwarding loops and packet duplication problems.

Last modified Nov 23,1999