An administrator is creating new severs in a public cloud. The workflow creates and application group with three VMs, one web server, one application server and one database server. The three VMs are

A misconfigured VLAN would block all inter-VM communication including SSH, but SSH is confirmed working to all three VMs, eliminating VLAN misconfiguration as the cause.

A misconfigured proxy affects HTTP or HTTPS forwarding at the web tier, not backend database connectivity, and would not produce a 503 error referencing a database issue.

While a firewall rule blocking the database port is plausible, automated cloud workflows that create the VMs and security groups together are more likely to misconfigure routing between subnets than to create an intentionally open SSH rule while blocking the database port.

Public cloud environments place web, application, and database tier VMs in separate subnets, each governed by its own route table. SSH may succeed over a management path while application-to-database traffic fails if the route table for the application subnet lacks an entry directing database-bound traffic to the correct next hop. A misconfigured route table is the most common oversight in automated cloud workflows that create subnet-separated tier architectures, producing a 503 error when the app server cannot reach the database.