Let's analyze each option to find the one that meets the requirements of no startup time for new traffic, two idle instances, and minimal administrative overhead:
A: Unchecking "Serve this revision immediately" and using a traffic simulation tool: Unchecking "Serve this revision immediately" does prevent the new revision from receiving traffic immediately. However, manually using a traffic simulation tool adds administrative overhead. It also doesn't guarantee that two idle instances will be ready before traffic is shifted; you would need to monitor and adjust traffic manually based on the simulation.
B: Configuring service autoscaling and setting the minimum number of instances to 2: Service-level autoscaling applies to all revisions of the service. Setting the minimum instances at the service level would ensure at least two instances are running across all active revisions, not specifically for the new revision before traffic shift.
C: Configuring revision autoscaling for the new revision and setting the minimum number of instances to 2:
This is the correct approach. By configuring revision autoscaling specifically for the new revision and setting the minimum number of instances to 2, Cloud Run will ensure that at least two instances of the new version are running and ready to serve traffic before you redirect any traffic to it. This eliminates startup latency when you do shift traffic. It also minimizes administrative overhead as Cloud Run manages the instance scaling based on this configuration.
D: Configuring revision autoscaling for the existing revision and setting the minimum number of instances to
2: This would ensure the existing version has at least two idle instances, which doesn't directly address the requirement of having idle instances ready for the new version before traffic redirection.
Google Cloud Documentation References:
Cloud Run Autoscaling: https://cloud.google.com/run/docs/configuring/min-instances - This document explains how to configure minimum and maximum instances for Cloud Run services and revisions. It clarifies that you can set minimum instances at the revision level to ensure instances are always ready.
Cloud Run Traffic Management: https://cloud.google.com/run/docs/managing/traffic - This describes how to deploy new revisions and gradually shift traffic between them. Combining minimum instances on the new revision with traffic splitting allows for zero-downtime deployments with pre-warmed instances.

The BGP multi-exit discriminator (MED) attribute is used in BGP configurations to influence the choice of path in an active/passive setup by prioritizing one path over another for egress traffic. This is ideal for a design that uses only one VLAN attachment at a time.

The SA_DELETE message in VPN logs, especially when followed by a re-establishment, is a strong indicator that the Security Association (SA) lifetimes for Phase 1 (IKE SA) or Phase 2 (IPsec SA) are mismatched between the Google Cloud HA VPN and the on-premises router. When one side's SA expires, it sends an SA_DELETE message to the peer, which then triggers a rekeying process. If the lifetimes don't match, one side might prematurely terminate the SA, leading to brief disconnections. Ensuring that the IKE parameters (specifically Phase 1 and Phase 2 lifetimes) match the recommended values in the Cloud HA VPN documentation is crucial for stable tunnel operation.
Exact Extract:
"If your VPN tunnel frequently disconnects and reconnects, and you see SA_DELETE messages in your logs, it often indicates a mismatch in the Phase 1 (IKE SA) and Phase 2 (IPsec SA) lifetimes configured on your on- premises VPN gateway and the Cloud VPN gateway."
"For optimal stability, ensure that the IKE lifetime and ESP/IPsec lifetime parameters on your on-premises VPN device exactly match the recommended values provided in the Cloud VPN documentation. Mismatched lifetimes can cause tunnels to rekey prematurely or fall out of sync, leading to temporary disconnections." Reference: Google Cloud VPN Documentation - Troubleshooting VPN issues, Recommended IKE and IPsec settings

When troubleshooting connectivity issues, especially over public internet connections with intermittent errors, Connectivity Tests in Network Intelligence Center are crucial. This tool allows you to simulate the connectivity and understand the data plane status of Google Cloud resources. Since ICMP tests pass but TCP tests fail intermittently, using Connectivity Tests with TCP parameters will provide detailed insight into possible network issues like route misconfigurations, peering issues, or other transient problems affecting only specific protocols.
Reference: Google Cloud - Network Intelligence Center
Reference: Google Cloud - Troubleshooting with Connectivity Tests