Below are the steps to be followed to attach the additional slaves to an existing kubernetes cluster.

Execute below commands in Kubernetes (new) Slave:

kubectl run -i --tty ubuntu --rm --image=ubuntu --restart=Never --overrides='
apt-get update && apt-get install -y apt-transport-https
curl -s https://packages.cloud.google.com/apt/doc/apt-key.gpg | apt-key add -
cat <<EOF >/etc/apt/sources.list.d/kubernetes.list
deb http://apt.kubernetes.io/ kubernetes-xenial main
EOF
curl -fsSL 'https://sks-keyservers.net/pks/lookup?op=get&search=0xee6d536cf7dc86e2d7d56f59a178ac6c6238f52e' | sudo apt-key add -
sudo add-apt-repository \
"deb https://packages.docker.com/1.13/apt/repo/ \
ubuntu-$(lsb_release -cs) \
main"
apt-get update
apt-get -y install docker-engine
Run docker as daemon in as given below (if required):
Open /lib/systemd/system/docker.service and add the below line and save the file:
ExecStart=/usr/bin/dockerd -H fd:// -H unix:///var/run/docker.sock -H tcp://0.0.0.0:2376 $DOCKER_OPTS
Execute the following commands to reload and restart docker
systemctl daemon-reload
systemctl restart docker
apt-get install -y kubeadm=1.9.4-00 kubectl=1.9.4-00 kubelet=1.9.4-00 kubernetes-cni
service kubelet start

To know the K8s cluster join token:

To get the join command, execute the below command in Kubernetes Master.

Command: kubeadm token create --print-join-command

Result: kubeadm join --token 8xxxxa.6axxxxda3xxx0fx4 <<master-ip>>:6443 --discovery-token-ca-cert-hash sha256:a54ab1234e247eeb36ba772eb442b507b0a1234f8b1234567df1234567890c01

Reference: https://linuxacademy.com/community/posts/show/topic/25431-tip-kubeadm-print-join-command

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