In several installations with VMWare vNIC drivers & Flannel, we have noticed a particular edge case requiring TCP checksum offloading disabled in the Linux kernel. This can result in seemingly random network issues between cluster nodes.
The following command disables the checksum offloading:
ethtool -K flannel.1 tx-checksum-ip-generic off
To make the change persistent, add the following to a new systemd unit flannel-ethtool.service:
[Unit]
Description=Disable vxlan checksum offloading for flannel.1
After=sys-devices-virtual-net-flannel.1.device
Requires=sys-devices-virtual-net-flannel.1.device
[Service]
Type=oneshot
ExecStart=/sbin/ethtool -K flannel.1 tx-checksum-ip-generic off
RemainAfterExit=yes
[Install]
WantedBy=sys-devices-virtual-net-flannel.1.device
See the Red Hat docs or a more concise article here for information about using systemd units.
The simpler, and preferred, answer is to update the kURL installer and Flannel as this is built in as of Flannel 0.24.4.