I think its worse actually, you say it was around hourly before, its doing it every 1200 seconds for me now (every 20 minutes). It seems like not every event causes a problem on my LAN, just the larger ones.
Whatever the cause, all things point to the mDNS being very broken in HE ever since it was added. People have issues with Hub Mesh reconnecting, Homekit issues, the .local domain resolving, and this issue. Everything mDNS related seems to have issues.
FWIW, I have only a C7 and I see a mDNS spike from it every 20 minutes or so. Wireshark showed me about 6300 mdns packets from my hub in 1.6 seconds.
Edit: This have me thinking. My speakers uses mDNS to stay grouped. Lately they are loosing their group configuration. I have to regroup them all quite regularely. I did not have to do this before. I wonder if this could be the cause.
I also have this affecting some of my streaming servics and online conferencing.
Having to firewall around the Hubitat because of a regression bug is an unexepected PITA.
If you reboot the hub daily it helps a lot, it gets worse the longer the hub is running.
Yeah I suspect this might be what is making my music streaming pause for a second every once and a while also.
Yup. I have HubInfo reboot my hub daily at 330am - mainly because of this.
Has the latest update helped or is it still too early to say?
I started a discussion over in beta while the 181 update was being tested and it was determined that nothing has changed with this situation.
I'm running v2.3.9.180 and getting the UDP flooding.
How's this for punctuality - every 20mins without fail:
2024-09-23 12:28:08 Deny 10.xx.xx.25 224.0.0.251 mdns/udp 5353 5353 VLAN-15 Firebox udp flooding 65 255 (Internal Policy) proc_id="firewall" rc="101" msg_id="3000-0148" duration="0" sent_bytes="65" rcvd_bytes="0"
2024-09-23 12:47:57 Deny 10.xx.xx.25 224.0.0.251 mdns/udp 5353 5353 VLAN-15 Firebox udp flooding 62 255 (Internal Policy) proc_id="firewall" rc="101" msg_id="3000-0148" duration="0" sent_bytes="62" rcvd_bytes="0"
2024-09-23 13:07:47 Deny 10.xx.xx.25 224.0.0.251 mdns/udp 5353 5353 VLAN-15 Firebox udp flooding 62 255 (Internal Policy) proc_id="firewall" rc="101" msg_id="3000-0148" duration="0" sent_bytes="62" rcvd_bytes="0"
2024-09-23 13:27:37 Deny 10.xx.xx.25 224.0.0.251 mdns/udp 5353 5353 VLAN-15 Firebox udp flooding 65 255 (Internal Policy) proc_id="firewall" rc="101" msg_id="3000-0148" duration="0" sent_bytes="65" rcvd_bytes="0"
2024-09-23 13:47:27 Deny 10.xx.xx.25 224.0.0.251 mdns/udp 5353 5353 VLAN-15 Firebox udp flooding 62 255 (Internal Policy) proc_id="firewall" rc="101" msg_id="3000-0148" duration="0" sent_bytes="62" rcvd_bytes="0"
Do you have the hub segregated so the flood does not reach all the other devices on the LAN?
I don't block it but it is throttled, hence the flooding msgs.
Since identifying this issue I have been restarting my hub more often and watching the mDNS spikes. This has improved my LAN performance, eliminated the audio issues on meetings, and eliminated my RDP issues.
New feature in 2.3.9.184
The mDNS restarts is what has been causing the huge spikes in traffic, even if you are not using HK or Hub Mesh (which both need mDNS).
Personally, I recommended that EVERYONE should turn this OFF (defaults to On).
If you are using the HomeKit integration and start seeing the dreaded "no response" afterwards, then you may need to turn it back on again. Turning this off should have no impact on HubMesh.
Settings > Network Setup
Here is my testing results:
Orange = 183 (same as 184) HK enabled, Bonjour Restarts Disabled
Blue = 182 (beta) HK Disabled
Pink = 180 (prior production version) HK Disabled
When did that appear.
I thought it was only for the people in France and parts of Canada..... 
I don't even know what it is or whether it should be off or on.
I'll turn it off as it seems you are quite clued up with all this network Mularkey. 
Just today - .184 release
Aahhhh. That explains that one then.
I'm not sure if you will know but I have 2 hubs meshed. Would you recommend on or off. Big ask I know because every set up is different but just wondering of what recommendation would be best.
I turned mine off, but I do not run Hub Mesh (just HomeKit), so I'm not sure there's a deciseive answer yet on effects to either HK or Hub Mesh-- some of us are just running with it off to see what (if anything) shakes loose & falls apart.
I have Hub Mesh enabled on all 5 of my Hubs but I don't actually use it. However, after updating them all to .184 a few hours ago, I turned OFF the mDNS restart. I just checked and all 5 hubs still show the other 4 hubs as connected. So there's no instant failure of Hub mesh as a result. 
Turn it off and monitor for any Hub Mesh issues. From what I know the mDNS restarts was added behind the scenes a while back only to help with the HomeKit integration. So turning it off should not impact Hub Mesh. mDNS will still be running, it will just not restart every 20 minutes (which should not be necessary).
I have a number of different hubs and services that support home kit on my network.
None of them cause the spikes in mDNS traffic (at least nothing I've seen in wire shark) that hubitat does when it restarts the service.
So I guess I don't understand why hubitat has to restart every 20 minutes when other systems don't. 
Seems like a weird Band-Aid instead of figuring out the root cause.
After 5 years around here, I am betting you do know... 
One day, after a lot of topics regarding the dreaded "No Response" in Apple's Home message, the Bigger Hammer was wheeled out and applied. Today, with those topics diminished, but still around, the option to "put the Hammer back in the shed" has been given.
This is ultra common pathway amongst programmers, and that's the part I imagine you know.
Yup, I raised this exact same point over in the beta thread.
Download the Hubitat app
