Hi Eddie,
From a vSphere perspective, there is no requirement for all ESXi management vmkernel interfaces to reside on separate management networks simply because they belong to different clusters. A single vCenter can manage multiple clusters whose ESXi hosts share the same management subnet, provided connectivity, routing, DNS, and security requirements are properly implemented.
That said, I'd consider a few design aspects:
- Operational isolation. Sharing a management network means both clusters depend on the same management infrastructure. While this simplifies administration, it also increases the impact of any management network outage.
- Security. If both clusters have different security or operational requirements, separate management VLANs may provide better segmentation and reduce the attack surface.
- Availability. Since you're already planning a vCenter HA deployment, ensure the management network provides sufficient bandwidth, low latency, and redundancy. Avoid introducing firewall bottlenecks or asymmetric routing between ESXi hosts and vCenter.
- vSAN considerations. Keep the vSAN, vMotion, and management traffic on separate VMkernel adapters and VLANs. Sharing the management network does not imply sharing the vSAN network.
For VCF, the design guidance often shows separate management networks per workload domain because it promotes lifecycle isolation, scalability, and standardized deployments, not because a shared management subnet is technically unsupported. If both clusters are in the same datacenter, have similar trust boundaries, and are managed by the same operations team, a shared management VLAN is a perfectly reasonable design. If they're serving different business units, security zones, or compliance requirements, I'd lean toward separate management VLANs with routing through the firewall.
One question I'd like to ask is why Cluster A and Cluster B currently require different subnets if they're ultimately being managed by the same vCenter HA pair? Understanding the security or operational driver behind that decision would help determine whether maintaining separate management networks provides enough benefit to justify the additional complexity.