1
Diagnostic step 1
Temporarily disable IGMP snooping on the ISP modem if accessible and test whether discovery becomes stable (reversible via UI reboot).
FULL AI ANALYSIS
mDNS/Bonjour device discovery fails intermittently after ISP modem enables IGMP snooping
Generated Oct 3, 2026, 12:35:03 AM
Analysis 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.
Possible 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%mDNS packets crossing subnet/bridge without an mDNS reflector/AVAHI reflector configured
20%Suggested fixes
2
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.
3
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.
4
Diagnostic step 4
Capture simultaneous packet traces on client, OpenWrt, and ISP modem uplink during a discovery failure window (tcpdump -i any port 5353) to confirm where mDNS responses stop.
5
Diagnostic step 5
If modem configuration is locked by ISP, contact ISP support with packet captures showing IGMP group removals; escalate to a network engineer while avoiding any destructive changes to device configurations.