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Today the CF Routing team added the ability for an Envoy proxy deployed as the edge router for CF (using istio-release) to return a 503 instead of a 404 for a requested route when the mapped app is stopped or crashing. 

We are thrilled as this represents the first new routing feature in Cloud Foundry enabled by our next-gen routing subsystem based on Istio and Envoy. See the attached GIF for a micro-demo. 

While this is test-driven and CI'd functionality, I want to clarify that istio-release is considered experimental/alpha. 

For more on our initiative to leverage Istio and Envoy, see https://docs.google.com/document/d/1LgLY0g39fzpg1_4zTckbH1mOuuSKGvYwp2tkakoe9ys/edit