Network Pool Sizing Before Host Expansion
A VCF 5.0 instance uses one network pool. Management is 10.20.10.0/24, vMotion is 10.20.20.0/28, vSAN is 10.20.30.0/28, and NSX TEP is 10.20.40.0/28. The pool currently serves 13 hosts across the management domain and two workload domains. The customer wants to commission four more hosts into an existing workload domain next week. Each subnet uses .1 as its gateway. The physical switch ports for the new hosts are already configured with the correct VLANs. During a test commission of the first new host, SDDC Manager fails at IP allocation for vMotion. The customer asks whether they can simply add a second /28 range on a different subnet to the same pool.
1. Calculate the usable addresses in the /28 ranges and explain why the first new host fails. What must be expanded before commissioning?
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A /28 contains 16 addresses. Subtract the network and broadcast addresses, leaving 14 usable. Because .1 is the gateway, only 13 addresses remain for hosts.
The pool already serves 13 hosts, so the vMotion and vSAN /28 ranges are exactly full. The first new host needs a vMotion and vSAN address and fails because there is no free address. The TEP /28 also has no meaningful headroom once edge nodes are considered.
Before commissioning, expand vMotion, vSAN, and TEP to at least /27: 32 addresses, 30 usable, 29 after the gateway. That supports 17 hosts with room for replacements and edge nodes. Management /24 is not the constraint. Expand the pool and verify allocation before retrying commissioning.
2. Can the customer add a second /28 on a different subnet to the same network pool for the existing domain? What are the constraints?
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A VCF network pool can contain multiple IP ranges, but that does not mean a different subnet is safe for an existing domain. vSAN requires layer 2 reachability within the cluster, and vMotion and TEP design assumptions depend on routing and MTU. Adding a different subnet for vSAN addresses would break the cluster design.
The correct approach is to add a larger contiguous range in the same VLAN/subnet if the physical network can support it. If the subnet cannot be enlarged, create a new network pool and use it for a new workload domain rather than mixing subnets inside the existing vSAN cluster. Do not treat pool expansion as a purely administrative SDDC Manager change; validate switch VLAN, MTU, routing, and DNS first.
3. What prechecks would you perform before commissioning the four hosts?
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Verify free addresses in every required range for four hosts plus one spare per range, confirm the VLANs are trunked on the correct switch ports, and validate MTU 9000 on TEP and vSAN paths. Check DNS forward and reverse records, NTP reachability, host HCL compatibility, and that no duplicate IPs exist. Confirm the hosts are in the same L2 domains required by vSAN and vMotion, and that the network pool has not reserved addresses that overlap infrastructure. Only after these pass should the commissioning task be started.