mDNS/Bonjour device discovery fails intermittently after ISP modem enables IGMP snooping

Environment

Home network: ISP-provided modem/router (firmware v1.2.3), separate OpenWrt router in AP mode (v21.02), mixed Wi‑Fi and wired clients (macOS, iOS, Windows, Raspberry Pi running homebridge).

Symptoms

AirPlay speakers, HomeKit accessories and printers intermittently disappear from device lists on macOS and iOS. Disappearance lasts 30–120 seconds then devices reappear without rebooting clients.

Observations

Wireshark capture on wired LAN shows mDNS (UDP/5353) queries from clients with no immediate multicast responses during outage windows; subsequent captures show mDNS responses. Router logs from ISP modem show IGMP membership reports and entries removing a VLAN group at the same time as device disappearance. No packet loss on ICMP during windows; multicast group entries on OpenWrt unchanged.

Already attempted

Rebooted ISP modem and OpenWrt router; disabled client VLAN tagging; forced Wi‑Fi clients to 2.4GHz only; temporarily moved devices to same physical switch; enabled IGMP snooping on OpenWrt; captured pcap on both modem uplink and LAN. Issue correlated with upstream modem's IGMP group removals. Did not change ISP modem firmware.

Question

What diagnostic steps and reversible configuration changes can confirm whether the ISP modem's IGMP snooping is causing mDNS discovery failures, and how can mDNS traffic be reliably forwarded across the LAN without replacing the modem?

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AI DIAGNOSTICS

Incident analysis

Updated 1w ago• Trouble ID

Most likely causes

ISP modem's IGMP snooping/pruning aggressively removes multicast group state across LAN segments 90%

IGMP querier role flapping between modem and OpenWrt causing inconsistent membership maintenance 60%

Multicast packets blocked or fragmented by modem NAT/bridge implementation during bursts 30%

Summary

Symptoms point to multicast discovery (mDNS) being intermittently filtered or pruned at the ISP modem when it removes IGMP group state, preventing immediate responses to clients on other segments. The fact that devices reappear without client-side action and that OpenWrt shows steady group state suggests the ISP modem is the point where multicast state is lost. Multicast discovery relies on local LAN multicasting; IGMP snooping and querier behavior on devices between multicast sources and listeners can disrupt it if group state times out or is not propagated across link boundaries.

Suggested Fixes

Diagnostic step 1

Temporarily disable IGMP snooping on the ISP modem if accessible and test whether discovery becomes stable (reversible via UI reboot).

Diagnostic step 2

Enable a local mDNS reflector on OpenWrt (avahi-daemon reflector mode) to forward mDNS between interfaces and test impact; document config and revert if undesired.

Diagnostic step 3

Configure the OpenWrt router as the IGMP querier (if supported) and increase IGMP membership timeouts to prevent premature pruning; leave ISP modem in simple bridge mode if possible.