istio

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apiVersion: release-notes/v2
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kind: feature
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area: networking
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issue:
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- 27749
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releaseNotes:
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- |
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  **Updated** the default installation of gateways to not configure clusters for `AUTO_PASSTHROUGH`, reducing memory costs.
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upgradeNotes:
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- title: "`AUTO_PASSTHROUGH` Gateway mode"
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  content: |
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    Previously, gateways were configured with multiple Envoy `cluster` configurations for each Service in the cluster, even those
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    not referenced by any Gateway or VirtualService. This was added to support the `AUTO_PASSTHROUGH` mode on Gateway, generally used for exposing Services across networks.
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    However, this came at an increased CPU and memory cost in the gateway and Istiod. As a result, we have disabled these by default
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    on the `istio-ingressgateway` and `istio-egressgateway`.
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    If you are relying on this feature for multi-network support, please ensure you apply one of the following changes:
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    1. Follow our new [Multicluster Installation](/docs/setup/install/multicluster/) documentation.
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       This documentation will guide you through running a dedicate gateway deployment for this type of traffic (generally referred to as the `eastwest-gateway`).
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       This `eastwest-gateway` will automatically be configured to support `AUTO_PASSTHROUGH`.
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    1. Modify your installation of the gateway deployment to include this configuration. This is controlled by the `ISTIO_META_ROUTER_MODE` environment variable. Setting this to `sni-dnat` enables these clusters, while `standard` (the new default) disables them.
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      {{< text yaml >}}
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      ingressGateways:
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       - name: istio-ingressgateway
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         enabled: true
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         k8s:
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           env:
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             - name: ISTIO_META_ROUTER_MODE
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               value: "sni-dnat"
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       {{< /text >}}
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