Shortest-Path Tree (SPT) and Shared Tree

IP Multicast constructs shortest-path tree (SPT) and shared tree distribution paths to forward multicast packets to members of a group.
After a receiver joins a multicast group, the routers in the multiaccess network build the routing paths necessary to send data to each receiver in the group. Each IP datagram sent to a multicast group is distributed (forwarded) to all members. The routing paths constitute a type of distribution tree for a multicast packet. The goal of a multicast distribution tree is for the router to duplicate a multicast packet when the packet reaches a divergence of paths and the router must send the packet down multiple paths to reach all group members, yet the distribution tree must refrain from sending packets down a path where no interested receivers exist. The distribution tree is one of the following:
  • A source tree—A path from a multicast source (the root of the tree) through the network to the receivers in the multicast group. The source tree is the shortest path that a multicast packet can take from source to receiver, so it is also known as the shortest-path tree (SPT). The sender and receiver are annotated as a source and multicast group pair, shortened to (S, G); for example, (192.168.1.1, 225.9.2.6). The following figure illustrates three shortest-path trees from the source to three receivers.
    multicast_spt.png
  • A shared tree—A path rooted at the RP, not at the multicast source. A shared tree is also known as an RP tree or RPT. Routers forward multicast packets from various sources to the RP and the RP forwards the packets down the shared tree. A shared tree is annotated as (*, G), using a wildcard as the source because all sources belonging to the multicast group share the same distribution tree from the RP. An example shared tree annotation is (*, 226.3.1.5). The following figure illustrates a shared tree from the root at the RP to the receivers.
    multicast_shared_tree.png
Source-Specific Multicast (SSM) uses source tree distribution. When you Configure IP Multicast to use Any Source Multicast (ASM), you can specify which distribution tree the virtual router on your Palo Alto Networks® firewall uses to deliver multicast packets to a group by setting an SPT threshold for the group:
  • By default the virtual router switches multicast routing from shared tree to SPT when it receives the first multicast packet for a group or prefix (the SPT Threshold is set to 0).
  • You can configure the virtual router to switch to SPT when the total number of kilobits in packets arriving for the specified multicast group or prefix at any interface over any length of time reaches a configured number.
  • You can configure the virtual router to never switch to SPT for the group or prefix (it continues to use shared tree).
SPT requires more memory, so choose your setting based on your multicast traffic level to the group. If the virtual router switches to SPT, then packets will arrive from the source (rather than the RP) and the virtual router sends a Prune message to the RP. The source sends subsequent multicast packets for that group down the shortest-path tree.

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