If smart-home devices stopped working after a VLAN change, check two separate things: whether the device has a valid address on the intended network, and whether the controller can both discover and reach it. mDNS discovery is link-local multicast, so ordinary routing usually will not carry it across VLANs; even when discovery works through a relay, firewall or isolation rules can still block control.
1. Confirm the device joined the right network and received an address
Start with the device itself, not the discovery settings. Confirm it is powered on and associated with the intended Wi-Fi SSID or wired VLAN. Then check the router’s DHCP lease list for its current IP address, subnet, and gateway.
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Compare that information with the network the device is supposed to use. If the lease is missing or the address, subnet, or gateway is unexpected, investigate Wi-Fi association, DHCP, VLAN assignment or tagging, and gateway configuration first. A discovery relay cannot fix a device that has not joined the network or received a usable address. Home Assistant likewise recommends comparing discovered IP information with DHCP leases and checking reachability in its HomeKit Device troubleshooting guidance.
2. Check discovery from the controller’s network
mDNS (multicast DNS), commonly used for local service discovery, uses UDP port 5353 and multicast addresses 224.0.0.251 for IPv4 and FF02::FB for IPv6, as defined in RFC 6762. Because this traffic is link-local, ordinary routing does not normally make advertisements visible on another VLAN. Apple’s Bonjour deployment guide describes Bonjour discovery as multicast service advertising and notes that multicast is usually not routed between subnets.
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For Home Assistant
Open Settings > System > Network > Zeroconf Browser to see devices found through Zeroconf/mDNS. For HomeKit accessories, Home Assistant also recommends a diagnostic such as python3 -m netdisco; run it from a network context appropriate to the controller, since a phone on another network may see different results. The HomeKit Device guidance specifically calls for an mDNS reflector when Home Assistant and the HomeKit device are on different VLANs.
For other controllers
Use the controller’s discovery view or a service browser running on the controller host or network. Record whether the needed service appears and which address it advertises. If it does not appear across the VLAN boundary, check whether the gateway offers mDNS reflection or a Bonjour gateway, and whether any service filter permits the required service type. Home Assistant’s Zeroconf documentation explains that interface selection follows its Network integration and that a misconfigured host can prevent Zeroconf-based integrations from responding.
3. If discovery works, test the control path separately
A relayed advertisement tells the controller where a service claims to be; it does not grant permission to connect to it. Compare the advertised address with the device’s DHCP lease, then check the controller’s route, subnet masks, and gateway. Review firewall or access-control rules for the traffic the specific integration requires, including any return traffic it needs.
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There is no single smart-home control-port list that applies to every accessory and controller. Use the documentation for the particular device and integration to identify the actual ports and protocols rather than opening broad access between VLANs. If Home Assistant’s discovery tools find a HomeKit device but the integration still cannot use it, continue with its HomeKit troubleshooting guidance. Pairing state can also matter: Home Assistant says an accessory already paired elsewhere may need to be reset according to its manufacturer’s instructions before it can be added to that integration.
4. Review isolation and multicast settings
Check the controls that can prevent devices on separate networks—or even on the same Wi-Fi network—from communicating:
- Wi-Fi client or SSID isolation.
- Switch ACLs or device-isolation settings.
- Multicast and broadcast filtering or forwarding.
- IGMP or other multicast-handling settings.
For example, Ubiquiti’s device-isolation guidance says isolation blocks direct device-to-device communication, including discovery and casting. Its broadcast-traffic guidance recommends using mDNS relaying only where needed between a device VLAN and the clients that need to discover it.
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Home Assistant notes that network configuration, including IGMP snooping in some reported environments, has been associated with HomeKit discovery problems, while emphasizing that results depend on the hardware and configuration. Treat an IGMP change as a targeted test, not a universal fix. Change one relevant setting at a time and retest discovery and control so you can identify which change mattered.
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5. Check the controller’s interfaces and non-mDNS cases
Confirm the controller host, container, or virtual machine is connected to the intended interface or VLAN, has a route to the device network, and is listening for discovery on the right interface. Multiple network interfaces, host firewalls, and virtualization or container networking can affect multicast reception; check the platform-specific network configuration. Home Assistant describes interface selection in its Zeroconf documentation.
Not every smart-home device relies on mDNS: some use a different discovery or transport path, including Bluetooth or Thread. A missing mDNS advertisement alone does not establish that the device is offline. For a HomeKit accessory previously used with Thread, Home Assistant notes that stored Thread credentials can affect joining; resetting it may ensure it is not still tied to an earlier Thread network. Follow the HomeKit Device guidance and the manufacturer’s reset instructions for the specific accessory.
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6. Choose the narrowest fix that restores the needed path
Use the failure you observed to decide what to change. Discovery and control are separate requirements, so avoid opening more traffic than the working setup needs.
| What you observe | What to investigate | Next step |
|---|---|---|
| No DHCP lease, or an unexpected address, subnet, or gateway | Association, DHCP, VLAN assignment or tagging, and gateway configuration | Restore correct network membership and addressing before troubleshooting discovery. |
| Valid lease, but the service is not visible across VLANs | mDNS or other discovery method, relay scope, service filters, multicast handling, and controller interface | Check whether the device actually uses mDNS; if it does, configure a suitable relay for the intended networks and service. |
| Service is visible, but the controller cannot operate the device | Advertised versus leased address, routing, firewall rules, required protocol and ports, and isolation | Allow the integration’s documented control traffic rather than assuming discovery makes control possible. |
| Some devices or services work, but others do not | Per-service filters, device-specific transports, VLAN policies, and controller network placement | Test the affected service and device individually before broadening a relay or firewall rule. |
Check whether the existing gateway can reflect the required mDNS services between only the relevant networks and route the permitted control traffic. As one vendor-specific example, Ubiquiti documents mDNS forwarding between selected networks, with service filtering, and says that feature requires a UniFi Gateway. That is not a universal hardware requirement: capabilities and setting names vary by vendor and model. Consider different equipment only if the current gateway cannot provide the VLAN routing and narrowly scoped discovery relay your setup needs.
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