1
Diagnostic step 1
Audit MAC address allocation: in vCenter, check all MAC pools and ensure ranges do not overlap across clusters. If using multiple MAC pools, document mapping to clusters and reconcile any overlaps.
FULL AI ANALYSIS
ESXi vMotion causes guest VM to acquire a different MAC, triggering duplicate MAC and DHCP conflicts
Generated Oct 3, 2026, 12:35:03 AM
Analysis summary
VM MACs in vSphere should remain stable across vMotion. A change implies that either the VM's configuration on the destination host was modified at migration time (e.g., addressType changed/overridden), MAC pool allocations overlapped or vCenter/host applied a new generated MAC because the original was considered invalid or already assigned elsewhere. Another possibility is that a template or clone configuration with transient MACs was applied, or the VMX was inconsistent between hosts. Manual MAC assignment is the most reliable way to enforce a stable MAC but requires inventory changes.
Possible causes
vCenter/ESXi MAC pool allocation policy or configuration has inconsistent ranges across clusters, causing vCenter to reassign a generated MAC on destination.
85%VM configuration divergence (corrupt or inconsistent .vmx) leading destination host to recreate NIC with a new generated MAC.
40%Automated processes or orchestration tooling that reconfigures VMs during/after migration (cloud-init, guest customizers, orchestration hooks).
20%Third-party host-level features (e.g., network recovery scripts, PCI passthrough, SR-IOV) that force NIC recreation and thus a new MAC on the destination.
10%Suggested fixes
2
Diagnostic step 2
For critical VMs, set a static MAC reservation in the VM configuration (edit VM settings -> MAC address -> Manual) and document this as an inventory item. This prevents vCenter from regenerating a MAC. Test on one VM first; the change is reversible by reverting to 'Generated'.
3
Diagnostic step 3
Collect logs for a reproducer: capture vmware.log from both source and destination hosts around a vMotion, and hostd/vpxd logs from vCenter. Use these when escalating to VMware support if internal audit doesn't reveal the cause.
4
Diagnostic step 4
Temporarily pin critical VMs to a host (disable DRS for those VMs) if immediate stability is required while you diagnose. Avoid broad disabling of vMotion; prefer targeted host affinity rules.
5
Diagnostic step 5
If you suspect orchestration tooling or post-migration hooks, review automation jobs and vCenter alarms that run on vMotion. Revert or test these jobs in a staging environment before re-enabling. Escalate to VMware support when vmware.log shows unexplained MAC reassignment or if MAC pool audit does not reveal overlap.