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Slow Overlay Network?
Slow network connectivity using Overlay Networks in your swarm clusters using VMs for node? GRO(Generic Receive Offload) or RSC (Receive Segment Coalescing) might be to blame.
If you are running Virtualised Linux with latest 5+ Kernel for your Swarm nodes you may run into this issue.
You need to check GRO / RSC settings on your Hypervisor's network interface or vSwitch. Relevant issues reported at moby/moby here and here.

Linux Based Hypervisor's

Run the following command to get the status of GRO for your network interface
ethtool -k <interface name> | grep generic-receive-offload
You can turn GRO off by running the following command
ethtool -K <interface name> gro off
Repeat for all required interfaces. This should improve the network speeds for for your overlay networks between swarm node VMs

Hyper-V

Run the following command to get the status of RSC for your network interface
Get-VMSwitch -Name <vSwitchName> | Select-Object *RSC*
You can turn RSC off by running the following command
Set-VMSwitch -Name <vSwitchName> -EnableSoftwareRsc $false
Repeat for all required interfaces. This should improve the network speeds for for your overlay networks between swarm node VMs.
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