The Wi-SUN mesh is designed to scale across thousands of devices in large outdoor areas. It uses a dynamic, self-healing routing system that keeps data moving efficiently, even when conditions change.

For most users, this routing works automatically with no manual setup. However, understanding the principles can help when deploying networks, optimizing performance, or diagnosing communication issues.

Mesh topology

Wi-SUN networks use a multi-hop mesh topology, where each device can forward packets for others. This enables communication between devices that are out of direct radio range and provides extended coverage, redundancy, and reliability.

Unlike star-topology networks that depend on a single central access point, a Wi-SUN mesh distributes routing responsibility among all participating nodes. The Border Router acts as the root of the network, but routing decisions are made collaboratively by devices, creating a self-organizing and resilient structure.

Network formation

When a Wi-SUN network starts, the Border Router initializes the mesh and advertises its IPv6 network prefix so devices can join.

  1. The Border Router receives its global IPv6 address depending on its configuration and upstream interface behavior:

    • Prefix Delegation Mode : When enabled, and if the upstream network (Ethernet or Wi-Fi) provides a valid prefix through DHCPv6-PD, the Border Router uses that delegated prefix as its global address. It then advertises this prefix into the mesh for node assignment.

    • Static Prefix Mode (default): If no delegated prefix is available, the Border Router uses a static ULA prefix (typically fd11::/64) to construct its own global address and advertise it to the mesh. This configuration does not depend on external IPv6 services.

  2. Joining devices (e.g., XBee for Wi-SUN) use this prefix, together with their unique identifier (EUI-64), to form or request an IPv6 address through Dynamic Host Configuration Protocol for IPv6 (DHCPv6).

  3. After authentication, nodes initiate a DHCPv6 Solicit request to obtain their global IPv6 address. Then, they begin participating in routing through the RPL protocol.

This process establishes a consistent addressing domain across the network and enables end-to-end communication once routing becomes active.

Routing protocol: RPL

Routing in Wi-SUN is managed by the Routing Protocol for Low-Power and Lossy Networks (RPL), a standardized IPv6 routing protocol designed for constrained devices and unreliable links.

RPL uses link-local addresses for control messaging and the global IPv6 address space for routed data traffic.

RPL builds a tree-like routing structure called a Destination-Oriented Directed Acyclic Graph (DODAG), where:

  • The Border Router is the root node of the network.

  • Each node calculates its rank — a metric representing its distance (in terms of routing cost) from the root. Lower rank means closer to the root.

  • Nodes select one or more parent devices for forwarding data, based on rank and link quality.

  • The objective function defines how nodes choose their preferred parent, often using signal strength, link reliability, or hop count.

  • Each node learns its next-hop routes dynamically through RPL control messages and neighbor information.

This design ensures stable, loop-free routing and allows the network to reorganize automatically as link conditions evolve.

In Wi-SUN, RPL runs transparently in the background. Users typically don’t need to configure routes manually, but understanding its behavior helps with network design and optimization.

Traffic direction

Wi-SUN supports two routing directions:

  • Upstream (Node to Border Router): Packets originating from a node and destined for external networks are routed via the DODAG to the Border Router. Common for telemetry data or sensor readings sent to the cloud.

  • Downstream (Border Router to Node): Packets from the upstream network are routed to nodes using RPL’s downward routing (via source routing headers or hop-by-hop forwarding). Used for control messages, configuration, or over-the-air updates.

RPL is optimized for efficient upstream routing, but also supports downstream communication using route advertisements and caching mechanisms.

To prevent routing loops, RPL enforces strict rank ordering — data can only flow from higher to lower ranks.

Dynamic routing and self-healing

Wi-SUN meshes maintain stability through continuous route evaluation and automatic recovery. Each node monitors link quality and may adjust its parent relationships when conditions change, allowing the network to remain stable and energy-efficient without user intervention.

This adaptive, self-healing behavior ensures that communication continues even when devices go offline or environmental conditions fluctuate.

For an in-depth explanation of route optimization, stability, and recovery mechanisms, see Routing dynamics.