Join Kubernetes Worker Node to an existing Cluster:

Before you begin 

1. Two working node culesters are needed.
   
2. You need to have a working Kubernetes cluster – configured and working control plane node
3. Docker, Kubernetes tools(kubeadm and kubelet) and CNI plugins(CALICO, flannel) should installed in your Worker node
4. If using Firewall such as firewalld, ports 10250, 30000-32767 and ports required by your Pod network add-on should be opened in the firewall.
5. SSH access to the machine to be added
6. Configured kubectl for checking if the node is available in your cluster
7. These standard requirements are covered in detail in our Kubernetes cluster installations. 

Generate new token to join work node :
Use command below to generate the join command for work node.
 
kubeadm token create --print-join-command
 
output:

[root@SERVER-worker ~]# kubeadm token create --print-join-command
kubeadm join 192.19.74.111:6443 --token syjqnu.0q6piceqoxxfxiui --discovery-token-ca-cert-hash sha256:fa9bb688d19eecbda4db75bff49416282918d126c9bd94100066f06d4b3a780e
 

 Yellow highlighted is a command to join this node to the master node, copy this command, and execute this on master node server.

Now go to the master node server, and execute the join command.

Output will be:
[preflight] Running pre-flight checks
[preflight] Reading configuration from the cluster...
[preflight] FYI: You can look at this config file with 'kubectl -n kube-system get cm kubeadm-config -oyaml'
[kubelet-start] Downloading configuration for the kubelet from the "kubelet-config-1.17" ConfigMap in the kube-system namespace
[kubelet-start] Writing kubelet configuration to file "/var/lib/kubelet/config.yaml"
[kubelet-start] Writing kubelet environment file with flags to file "/var/lib/kubelet/kubeadm-flags.env"
[kubelet-start] Starting the kubelet
[kubelet-start] Waiting for the kubelet to perform the TLS Bootstrap...
 
This node has joined the cluster:
* Certificate signing request was sent to apiserver and a response was received.
* The Kubelet was informed of the new secure connection details.
 
Run 'kubectl get nodes' on the control-plane to see this node join the cluster.

After that Wait for some time to get Nodes “Ready”. below is the command to check  the status of worker node. all the nodes attached with master node will start showing here.

watch kubectl get nodes