IPTV Multicast Freezing on MS120-48 – Any Optimization Advice?

[GaeMi]

Hi everyone,

I'm running into an issue with IPTV multicast traffic on a Meraki MS120-48 switch and would really appreciate any insight or advice from the community.

Issue Summary:

1. IPTV set-top boxes experience frequent screen freezing when connected through the MS120-48.
2. Only a single multicast stream is tested, yet freezing still occurs.
3. IGMP Snooping is ON, and Unknown Multicast Flood is OFF — this combination works best but doesn’t resolve the issue.
4. All other ports are disconnected during testing (no background traffic, a one receiver and a one sender are connected and only send a one multicast stream from sender)
5. The multicast stream is Full HD (1920x1080), H.264 codec, 6 Mbps fixed bitrate.
6. When analyzing link status, I found that the IPTV set-top box always negotiates at 100 Mbps, while the multicast sender (encoder) negotiates at 1 Gbps on all switches.
7. Interestingly, on the MS120-48, I manually set the encoder port to 100 Mbps, and the freezing issue slightly improved — but it’s still significantly worse than when using a Cisco 9200L or Zyxel 1920G switch.
8. On the Zyxel 1920G and Cisco 9200L switches, even with multiple multicast streams and the same port speeds (1G ↔ 100M), the issue does not occur at all.

Questions:

1. Has anyone experienced similar multicast issues with the MS120-48?
2. Is there any known limitation in how the MS120-48 handles multicast internally (e.g., software vs hardware forwarding)?
3. Any QoS, VLAN, or other configuration tweaks that helped in your case?

Thanks in advance — any shared experience or tips would be greatly appreciated!

[Brash]

I don't have much experience with multicast on Meraki devices, but just thought I'd share the following in case you hadn't seen them. They're very good write ups of both best practices and troubleshooting.

https://documentation.meraki.com/MS/Other_Topics/Layer_2_Multicast_and_Troubleshooting

https://documentation.meraki.com/MS/Other_Topics/Multicast_Overview%2C_Configurations%2C_and_Best_Pr...

[cmr]

What firmware are you running and which ports are the devices plugged into?  17.2.1 resolves a multicast issue with ports 25-48.

[cmr]

It also resolves an issue where link-local multicast 224.0.0.x traffic isn't forwarded when flood unknown multicast traffic is disabled...

[PhilipDAth]

Is this just a single switch or single switch stack environment, or are there multiple MS120s?  Multiple switches will require an IGMP querier to be configured.

[GaeMi]

Hello everyone,

Thank you for all your suggestions. Unfortunately, I had already tried most of what was recommended.

I upgraded the firmware to 17.2.1, tested the switch using ports 1–24, and confirmed that the IGMP querier was functioning correctly. From a network configuration perspective, I believe everything is set up properly.

However, the freezing issue only occurred when I was testing with the MS120-48.

Fortunately, I learned something interesting from a Meraki TAC engineer. He explained that Meraki switches may not have adequate buffer settings by default to handle large or frequent multicast traffic efficiently. He also mentioned that only Meraki TAC engineers can adjust these buffer values manually.

So, I provided him with the switch names and serial numbers so he could apply the necessary configuration changes.

I’ll be testing the system again this Wednesday, and I’ll make sure to share the results here afterward.

[Brash]

That's an interesting point. I wouldn't expect the MS120 to have particularly deep buffers, but I didn't realize they may be customizable in the backend.

Keep us updated with the results 👍

[GaeMi · Accepted solution]

After the TAC engineer applied the buffer configuration, the multicast stream started working properly!