Forward mdns Traffic 224.0.0.251 same VLAN on GS1900-24HP

Good morning everyone,

I am currently having trouble getting mDNS (e.g., for Matter/HomeKit) to work reliably in my network and would appreciate your support.

Setup:

• Switch: Zyxel GS1900-24HP
• Multiple WLAN access points NWA220AX-6E, connected to the switch (ports 1–3)
• WLAN clients (including an iPhone) in VLAN 64 (10.0.64.0/20)
• Matter Bridge also in VLAN 64, connected to the switch (port 24)
• VLAN configuration:
  • AP ports: VLAN 64 tagged
  • Matter Bridge port: VLAN 64 untagged, PVID 64

The goal is to enable mDNS (224.0.0.251 / _matter._tcp.local, etc.) between WLAN clients and the Matter Bridge.

Observed behavior:

• The iPhone sends mDNS queries (confirmed via Wireshark by Port Mirroring , e.g. _matterc._udp.local)
• The Matter bridge is not discovered
• No relevant traffic is seen on the Matter Bridge(tcpdump -i eth0 port 5353)

Analysis (Port Mirroring):

I used port mirroring on the switch to narrow down the issue:

1. Mirror Port 2 (AP port) → PC (Port 5)
   → mDNS queries from the iPhone are visible
2. Mirror Port 24 (Matter Bridge) → PC (Port 5)
   → no mDNS traffic visible
3. Mirror Port 1 (another AP) → PC (Port 5)
   → also no mDNS traffic from the iPhone visible

Steps already taken:

• Disabled/Enable IGMP Snooping → no change
• Verified VLAN configuration → consistent
• Checked AP configuration (isolation, multicast, etc.) → no obvious issues, Traffic left the AP, can the th e traffic on Switch Port 2

Current assumption:

The mDNS traffic reaches the switch (visible on the AP port), but is not forwarded to other ports within the same VLAN (e.g., port 24).

This suggests a potential issue with multicast or link-local multicast handling (especially 224.0.0.251) on the switch.

Questions:

• Is this behavior known or expected for the GS1900?
• Is there a way to reliably flood link-local multicast (224.0.0.0/24)?
• Are there hidden settings (e.g., via CLI) that affect this behavior?

Thank you very much for your help!