This section explains Tree Segment Identifier (Tree-SID), which are used to enable efficient multicast distribution across segment-routing networks by assigning unique labels and defining specialized node roles.
A Tree-SID is a method for implementing tree-like multicast flows over a segmented routing network that:
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assigns a single SID label to represent all segments and devices within a multicast tree,
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uses an SDN controller (such as a device running SR-PCE with PCEP) to calculate the tree structure, and
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designates a specific role to each node within the tree for forwarding multicast data.
The SDN controller constructs routing paths using any constraints defined by network architects.
A key use case for constraint-based Tree-SID is when routers are configured to deliver two point-to-multipoint (P2MP) streams carrying the same content over different paths. In this scenario, the multicast flow is forwarded twice, with each copy following a unique path through the network. The two copies use different nodes and links to reach the destination. This approach reduces packet loss because a network failure on one path does not impact the delivery of the multicast stream on the other path.
For detailed information on Tree-SID, see the Segment Routing Tree-SID configuration documentation for your specific device (for example, Segment Routing Configuration Guide for Cisco NCS 540 Series Routers).
Tree-SID node roles
Crosswork Network Controller provides the ability to view the details of Tree-SID root, transit, leaf, and bud nodes in the UI, allowing you to easily confirm proper Tree-SID implementation in your network (see View point-to-multipoint trees on the topology map).
Tree-SID policies define four distinct node roles, each of which serves a specific function in multicast data forwarding:
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Root: The ingress or head-end node. Encapsulates multicast traffic, replicates it, and forwards it to the transit nodes.
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Transit: Acts as a leaf (egress) node and a mid-point (transit) node toward downstream sub-tree.
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Leaf: The egress or the destination node. Decapsulates multicast traffic and forwards it to multicast receivers.
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Bud: Has a separate leaf node path and is displayed separately in the topology map.
Tree-SID policy types
Crosswork Network Controller allows you to visualize these Tree-SID policy types.
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Static: Configured via SR-PCE, either directly using SR-PCE CLI or through the Crosswork Network Controller UI. For more information and supported configuration commands, for your specific device, refer to the Tree-SID configuration documentation (for example, Segment Routing Configuration Guide for Cisco ASR 9000 Series Routers.
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Dynamic: Not explicitly configured. It is configured as part of an L3VPN or multicast VPN (mVPN) service.
You can use either static or dynamic Tree-SID policies to enable fast reroute (FRR) capabilities in your multicast network.