I'm running into the same issue. This seems to be a somewhat common issue with USB Type-C hubs.

Pass through power USB Type-C Hubs cause broadcast storms

Hi,

thanks for your reply.

Out of curiosity, do you connect a Dell laptop or another brand via USB-C?

Is the Laptop connected when it happens?

In my case the Laptop is always connected and I have the feeling that it happens more often with my Mac than the XPS - but it‘s just a feeling.

Best regards,

Stefan

I've only tried this with my MacBook Pro but it happened consistently, about a minute after putting the computer to sleep.

Hi,

thank you for your reply.

Yes, with the Mac connected I don't have to wait long until it happens.

I have a MacBook Pro 13 late 2016.

The following article describes exactly my problem:

https://lucumr.pocoo.org/2020/7/6/usb-c-network-hubs/

I debugged the problem further and see the following:

- The U3421's USB-C dock starts sending PAUSE frames after the Mac enters sleep mode

- My desktop switch where the monitor is connected to does forward the PAUSE frames which is a violation of the 802.3 Ethernet standard. Because of this the network is flooded with PAUSE frames leading to unreachable devices

- Waking up the Mac or pulling the network cable brings back the network

Because my desktop switch does forward the PAUSE frames I can capture them on another device (Raspberry in my case) using tcpdump:

# On the Raspberry connected to same desktop switch as the monitor - Mac is in sleep
# sudo tcpdump -i eth0 ether dst 01:80:C2:00:00:01
[...]
18:09:13.357831 MPCP, Opcode Pause, length 46
18:09:13.525607 MPCP, Opcode Pause, length 46
18:09:13.693380 MPCP, Opcode Pause, length 46
18:09:13.861122 MPCP, Opcode Pause, length 46
18:09:14.028902 MPCP, Opcode Pause, length 46
18:09:14.196666 MPCP, Opcode Pause, length 46
18:09:14.364408 MPCP, Opcode Pause, length 46
18:09:14.532195 MPCP, Opcode Pause, length 46
18:09:14.699941 MPCP, Opcode Pause, length 46
18:09:14.867717 MPCP, Opcode Pause, length 46
18:09:14.904456 MPCP, Opcode Pause, length 46
[...]

My desktop switch is a D-Link DGS-108 Rev C6.

Best regards,

Stefan

Interesting, thanks for digging into it.

I imagine this is a hardware issue that Dell can't fix with a firmware update?

And do you know of a workaround for a Mac that doesn't require running an ethernet cable directly to the laptop? I do have a spare Raspberry Pi that I can use.

I‘m not sure whether it can be fixed by Dell or Apple.

I have a workaround using the tool sleepwatcher. I disable the monitor‘s Ethernet adapter when sleep mode is entered and reenable it on wakeup. During sleep WiFi is used. This setup is stable now for a day. 
I already prepared a detailed post for this forum but got an error when I posted it. I‘ll try again tomorrow when I‘m back at my Mac. 

Here is the post I prepared today but the forum software just errors out when trying to post it.

Reason for the Dell forum error seems to be the shebang in the scripts pointing to bash shell. I removed it from the script examples below - they must be added to run the scripts. 

####

Hi,

I tried to disable e.g. PowerNap or network sharing without success and I'm not sure whether I can configure the network adapter in a way (e.g. on command line) to avoid this issue.

I tried another workaround which disables the monitors network adapter when entering sleep state and reenables it on wakeup. During sleep the Mac has to use WiFi which is acceptable for me.

First I renamed the network adapter in network settings to "Dell U3421WE".

I installed sleepwatcher (see https://www.kodiakskorner.com/log/258 ) and created two executable scripts:

# ~/.sleep
# Sleepwatcher script to be executed before sleep
# Disable network interface of Dell monitor to avoid sending PAUSE frames during sleep
networksetup -setnetworkserviceenabled "Dell U3421WE" off

# ~/.wakeup
# Sleepwatcher script to be executed after wakeup
# Enable network interface of Dell monitor
networksetup -setnetworkserviceenabled "Dell U3421WE" on

Now the adapter is disconnected once the Mac enters sleep. The monitor seems to keep the network link to the switch but so far did not generate PAUSE frames any more. I guess this will only work if the user is logged in but this is the case for me almost all the time. Maybe enabling/disabling the network interface can be controlled by the system agent of sleepwatcher but I haven't tried this yet.

Best regards,

Stefan

I'm having the exact same issue – it kills my entire network. My workaround for now is to connect the ethernet directly to my mac via a Belkin adapter, but I must say that's disappointing – one of this display's key selling points for me was the ability to hook everything up to it and use a single USB-C cable connected to the mac, and it's pretty bad to see such a problematic issue on a brand new product.

If Dell can find a fix, that would be hugely appreciated.

* Open the Menu- Display- USB-C Prioritization
* Change to High Data Speed
* Exit the Menu and retest