Define the boundaries before connecting the clouds
Before configuring a VPN or private circuit, put workloads, data, and ownership on the same diagram. Each cross-cloud flow should identify its source, destination, protocol, frequency, expected volume, sensitivity, and acceptable outage window. AWS and GCP should not be treated as one large LAN. They remain separate failure domains with independent identity systems, routing behavior, security controls, operational tooling, and billing models.
We normally define three boundaries first. The trust boundary determines where authentication, authorization, and encryption are enforced. The failure boundary assigns ownership when DNS, routing, or connectivity breaks. The billing boundary explains who pays for circuits, transferred data, and shared appliances. Private addressing is not a trust mechanism: cross-cloud services should still use TLS, with mutual TLS where stronger service identity is required, plus explicit routing and firewall policy on both sides.
Placement matters as much as connectivity. An application that crosses clouds for every database query acquires latency, transfer cost, and an additional runtime dependency. Keep highly interactive compute and data together when possible. Exchange events, batches, cached results, or asynchronous jobs across the boundary instead of treating a remote database as a local service.
Choose connectivity by traffic behavior, not bandwidth alone
The correct option depends on availability, latency consistency, throughput, encryption requirements, and the team's ability to operate it. A private circuit is not automatically the right production answer, and a quickly deployed VPN should not become permanent without review. Common patterns include:
- Public HTTPS APIs: A good fit for low-to-moderate traffic with narrow service boundaries. Strong workload identity, short-lived credentials, ingress protection, and deliberate timeout behavior can make this easier to govern than a broad private network.
- Highly available IPsec VPN: Useful for an initial deployment, a backup path, or workloads with uncertain traffic. Use redundant tunnels on separate endpoints or paths, then test tunnel loss, BGP convergence, and large packets.
- Carrier or partner cross-cloud connectivity: Appropriate for sustained transfer or more predictable latency. It adds port, attachment, carrier, and operational charges, and a private path does not necessarily provide end-to-end encryption.
- Replication or messaging: When the requirement is data movement rather than arbitrary IP reachability, object replication, queues, event buses, or managed pipelines can reduce the routable surface and failure scope.
Whichever pattern you choose, reserve non-overlapping CIDRs, limit advertised prefixes, and document BGP preference. High availability must cover gateways in both clouds, zones or regions, carrier paths, and physical locations. Two tunnels terminating on the same device or underlying circuit can still share a single point of failure.
DNS, return paths, and observability cause the subtle failures
After basic connectivity works, the first production issue is often intermittent timeout rather than total failure. Common causes include DNS returning an unreachable address, asymmetric return routing, an MTU mismatch that affects only larger packets, or NAT hiding the source identity expected by a firewall. Private DNS zones do not become mutually visible merely because networks are connected. AWS security groups and GCP firewall tags or service-account selectors also cannot be referenced as if they were shared controls.
- Create a dedicated cross-cloud DNS namespace with conditional forwarding, a documented authoritative side, and intentional cache lifetimes.
- Trace forward and return routes hop by hop. State which side may advertise a default route, and never assume peering or route propagation is transitive.
- Test path MTU discovery and MSS handling, particularly when VPNs, containers, and security appliances are combined.
- Enable flow logs, tunnel or interconnect metrics, DNS query logs, and application traces on both sides. Carry a common request identifier across the boundary.
- Alert on latency, packet loss, tunnel state, route changes, and transferred volume instead of checking only whether instances are running.
Pre-production exercises should disable one tunnel, withdraw a route, stop a DNS forwarding endpoint, and block traffic at one firewall. Observe existing sessions, retries, queue growth, and alert delivery. Proving that the happy path works is not the same as proving resilience.
Make billing boundaries match ownership and traffic direction
AWS commonly consolidates payment through accounts and Organizations, while GCP links projects to Cloud Billing accounts; these layers are related but not equivalent. Shared connectivity can live in a dedicated AWS network account or GCP network project, yet fixed charges, attachments, and data transfer might not be billed to the same owner. Depending on the product, cost attribution can follow the port, VLAN attachment, originating service, region, or resource-owning account or project. Reviewing only the central network bill will therefore miss part of the cost.
A useful cross-cloud estimate includes source-side data transfer out, transfer out in the response direction, inter-region movement, VPN or interconnect fixed charges, carrier circuits, NAT, transit routing, firewalls, load balancers, logging, and monitoring. Data transfer in is often uncharged, but that does not make a request free in one direction: responses, acknowledgements, retries, and regional detours can generate separate SKUs. AWS Savings Plans or reservations and GCP committed-use discounts also remain separate economic pools.
- Standardize cost-center, product, environment, and owner tags or labels, and enforce them when resources are created.
- Export detailed AWS cost and usage data and GCP billing data to a shared analytical model with consistent internal categories.
- Decide whether shared circuits use fixed allocation, usage-based allocation, or a central platform budget before invoices arrive.
- Configure budgets and anomaly notifications in both clouds. Ordinary budget alerts are not a universal hard spending cap.
- Review top cross-cloud flows, unallocated charges, regional detours, and idle connectivity with both network and finance owners.
A sound multi-cloud design does not make AWS and GCP appear identical. It gives every connection a purpose, every failure an owner, and every charge a traceable destination. If those answers are not visible in both the architecture diagram and the cost report, reduce the connectivity scope before expanding the deployment.