A detailed walkthrough of getting Matter over Thread devices working across VLANs in a segmented home network. Covers flashing a SONOFF EFR32MG21 dongle with OpenThread RCP firmware via Home Assistant's Silicon Labs Flasher, configuring the OpenThread Border Router add-on (including a hardware flow control fix), and solving the three cross-VLAN challenges: mDNS reflection with Avahi, routing Thread mesh ULA IPv6 prefixes via a fd00::/8 catch-all route to the HA VM's link-local address, and opening the right nftables firewall rules including ICMPv6 for Path MTU Discovery. The result is a phone on a client VLAN successfully commissioning and reaching Thread devices on the HA VLAN.
Table of contents
Matter and Thread, the short versionThe topologyFlashing the dongleThe border routerCrossing the VLAN boundaryCommissioningCounting the moving partsSort: