title: How to enable Istio SDS on existing GKE cluster
body: <p>I have an existing GKE cluster with the Istio addon installed, e.g.:</p>
  
  <pre><code>gcloud beta container clusters create istio-demo \
      --addons=Istio --istio-config=auth=MTLS_PERMISSIVE \
      --cluster-version=[cluster-version] \
      --machine-type=n1-standard-2 \
      --num-nodes=4
  </code></pre>
  
  <p>I am following <a href="https://medium.com/@gregoire.waymel/istio-cert-manager-lets-encrypt-demystified-c1cbed011d67" rel="nofollow noreferrer">this guide</a>  to install <code>cert-manager</code> in order to automatically provision TLS certificates from Let's Encrypt. According to the guide, Istio needs SDS enabled which can be done at the point of installation:</p>
  
  <pre><code>helm install istio.io/istio \
         --name istio \
         --namespace istio-system \
         --set gateways.istio-ingressgateway.sds.enabled=true
  </code></pre>
  
  <p>As I already have Istio installed via GKE, how can I enable SDS on the existing cluster? Alternatively, is it possible to use the <code>gcloud</code> CLI to enable SDS at the point of cluster creation?</p>