Multicast Distribution Trees

Source Trees or Shortest Path Trees(SPT).
  • A source tree implies that the route between the multicast source and receivers is the shortest available path, therefore, source trees are also referred to as shortest path trees (SPTs).
  • Source of Multicast Traffic is located at root of the tree. Receivers are located at the end of branches.
  • Multicast traffic flows from source down the tree towards the receivers using multicast forwarding table (show ip mroute). The multicast forwarding table consists of series of (S,G) entries, pronounced as S comma G, where S is a Source and G is the multicast group joined by receivers who are interested for multicast feed. Below diagram shows two source servers and sending the multicast traffic to receivers and respectively through a shortest path tree (blue and red arrows respectively).
  • A separate source tree exists for every source that is transmitting multicast packets, even if those sources are transmitting data to the same group. This means that there will be an (S, G) forwarding state entry for every active source in the network.
  • Examples of SPT are Source Specific Multicast (SSM). Another example can be be used from Shared tree while the receiver comes  to know about the Source through RP (first packet it receives from RP with S,G entry) and immediately sends PIM join to the Source using shortest path (SPT) in the network. This when (*,G) converges to (S,G)

Shared Tree

  • Shared trees differ from source trees in that the root of the tree is a common point  somewhere in the network, referred to as the rendezvous point (RP).
  • The RP is the point at which hosts/receivers join to learn of active sources. Multicast sources must transmit their traffic to the RP. When receivers join a multicast group on a shared tree, the root of the tree is always the RP, and multicast traffic is transmitted from the RP down toward the receivers.
  • An RP can be the root for all multicast groups in the network, or different ranges of multicast groups can be associated with different RPs.
  • Multicast forwarding entries for a shared tree use the notation (*, G), pronounced as star comma G. This is because all sources for a particular group share the same tree.  Therefore, the '*' represents all sources. The ultimate aim is to learn source using the RP and once the RP discovers both Source and Receivers, the tree gets complete. RP acts as a got-between that helps Receivers discovers the source and converting (*,G) entry to (S,G).
  •  Shared tree is insecure way of running multicast services because any source can send multicast to the receivers who have joined a particular group. The receivers are only interested for the multicast traffic for the group they have joined without considering the source identity which can be a hacker. 
  • Below Diagram shows the shared tree.  Receiver1 and Receiver 2 send (*,G) PIM join to RP (root or the tree). Also Source update RP about its presence using PIM 'Register' message. RP then updates source and group (S,G) details to last hop routers down the tree completing the tree. 
  • Examples of Shared tree are Any Source Multicast (ASM) using PIM Sparse-mode.