kubernetes 1.18 Stacked control plane and etcd nodes unable to add second ETCD node
I ran into a similar issue with v1.18.1, and root-caused it to the fact that my control-plane hosts had an incorrectly configured MTU value for the network interface. This was causing the etcdserver pod interactions between the first and subsequent control-plane nodes to timeout, because some network packets were getting silently dropped.
Fixing the MTU allowed me to complete the control-plane setup as advertised.
Details for my setup:
In my case, I was using KVM VMs (launched using LXD) as my control-plane hosts. Because of a DHCP misconfiguration error, the hosts were not receiving the proper MTU, so stayed with the defaul value of 1500 ... which ran into problems with the inter-host overlay network. Reducing the MTU down to 1450 resolved the issue.
If you are interested in additional details about why the MTU misconfiguration manifested in the way it did, I found the following Project Calico issue discussion useful:https://github.com/projectcalico/calico/issues/1709