Wi-Fi mesh on OpenWrt: setup and topology

Setting Up Wi-Fi Mesh Networks on OpenWrt: From Topology to Configuration

Wi-Fi mesh networks on OpenWrt use the 802.11s protocol to create self-organizing structures. Each node acts as a repeater, routing traffic through optimal paths. This differs from repeaters: in a mesh, data is transmitted only along the necessary route, minimizing load.

Physical topology determines device placement, while logical topology defines packet routes. Key types:

• Bus: Sequential connection, saves cable, but a break segments the network.
• Ring: Closed bus, resilient to single failures.
• Star: Each node connects to a central hub, isolating failures but requiring more cable.
• Mesh: Multiple connections, self-healing through alternative paths.

In wireless mesh, mesh at the L2 level combines with a star topology for clients. The backhaul is the wireless backbone between routers.

Designing Node Placement

Geometry matters more than transmitter power. If all nodes see each other, interference occurs: nodes compete for airtime via CSMA/CA, reducing throughput to a minimum (the "Mesh of Death" effect).

Optimal placement rules:

• Each router sees exactly two neighbors—maximizes airtime, bus topology.
• Close into a ring for fault tolerance and load balancing.
• Add a third neighbor at key points for redundancy.

Examples:

• 2 nodes: Any placement.
• 3 nodes: WAN router in the center.
• 4 nodes: In elongated spaces—linearly with WAN at 2nd/3rd; otherwise, first sees 2nd and 3rd, they see 4th.

Draw a diagram before installation, considering walls and interference.

802.11s Theory and Differences from Multi-AP

Mesh (802.11s) is not just an SSID on multiple access points. Nodes exchange management frames for discovery, routing (HWMP), and metrics. Supports multi-hop traffic.

Differences from controller-based systems (e.g., CAPWAP):

• Self-organization without a central node.
• Automatic reconfiguration during failures.
• Local traffic routing.

In OpenWrt, mesh is configured in /etc/config/wireless via wifi-device and wifi-iface sections with the mesh_id option.

Basic Mesh Setup in OpenWrt

On the main node (gateway):

config wifi-device 'radio0'
    option type 'mac80211'
    option channel '36'
    option hwmode '11a'
    option path 'pci0000:00/0000:00:00.0'
    option htmode 'HT80'
    option country 'RU'

config wifi-iface 'mesh_backhaul'
    option device 'radio0'
    option network 'lan'
    option mode 'mesh'
    option mesh_id 'my_mesh_net'
    option encryption 'sae'
    option key 'mesh_password'
    option mesh_fwding '1'

On leaf nodes similarly, but without WAN. All nodes share the same mesh_id.

For clients:

config wifi-iface 'ap_client'
    option device 'radio1'
    option mode 'ap'
    option ssid 'ClientSSID'
    option encryption 'psk2'
    option key 'clientpass'
    option network 'lan'

Separate radios: one for backhaul (5 GHz), another for clients (2.4 GHz).

Roaming and Parameter Optimization

Seamless roaming requires 802.11r/k/v. In OpenWrt:

• option ft_psk_generate_local 1 for FT.
• option ieee80211r '1'.
• option mobility_domain 'abcd' on all APs.

Key parameters:

• option max_neightbours 32—neighbor limit.
• option mesh_rssi_threshold 0—discovery threshold.
• option metric_mode 'default'—path selection (airtime).

Test RSSI: aim for -20 to -60 dBm for backhaul.

Scaling and Monitoring

Networks >10 nodes require VLAN for backhaul, QoS (cake/sqm). Monitoring: batman-adv over mesh or LuCI statistics.

Issues and solutions:

• Low speed—reduce neighbors, optimize channels.
• Failures—check ring topology.
• Overheating—dual-radio, ventilation.

Key takeaways:

• Place nodes by the "2 neighbors" rule for airtime efficiency.
• Use ring topology for fault tolerance.
• Separate radios: 5 GHz backhaul, 2.4 GHz clients.
• Enable 802.11r for seamless roaming.
• Monitor RSSI and airtime for optimization.

— Editorial Team