1
Diagnostic step 1
Modify restore automation to explicitly remove hwaddr entries from the container config before starting the container so Proxmox generates a new MAC (or use the restore flag that enforces unique MAC allocation).
FULL AI ANALYSIS
Proxmox LXC restores create containers with duplicate MAC addresses causing ARP flaps after backup imports
Generated Oct 15, 2026, 9:17:34 AM
Analysis summary
Most likely the backup includes the original container's hwaddr and the restore process is preserving it instead of generating a fresh MAC for the new target VMID. Another possibility is deterministic MAC generation based on VMID or node-specific seed (if VMID reused) causing collisions when restores are scripted. A reversible mitigation is to enforce clearing hwaddr during restore and let Proxmox assign a fresh MAC, or pass a flag to the restore tool to generate unique MACs. Also verify restore scripts do not blindly copy old config files into /etc/pve without adjusting hwaddr.
Possible causes
Restore process preserves original 'hwaddr' from backup config, resulting in duplicate MACs when multiple restorations of the same backup occur.
75%Proxmox predictable MAC generator returns same MAC because VMID was reused or the restore script forces a specific VMID that collides with existing assignments.
50%Cluster config database did not update MAC allocation on restore due to a temporary cluster quorum issue, allowing duplicate allocation.
25%External DHCP server/sticky MAC configuration maps an old MAC to a different port, producing apparent duplicates at the switch level.
15%Suggested fixes
2
Diagnostic step 2
If using scripted VMID reuse, stop reusing VMIDs for restores or ensure the MAC generator is seeded per-cluster so generated MACs will be unique; prefer letting Proxmox assign the MAC.
3
Diagnostic step 3
As a diagnostic, restore a single container and check /etc/pve/nodes/<node>/lxc/<vmid>.conf to confirm whether hwaddr is present; remove it and restart the container to confirm unique MAC assignment.
4
Diagnostic step 4
Temporarily clear the switch MAC address table entries for affected ports (or shutdown one side) to allow ARP to relearn correct mappings and reduce user impact while fixes are applied.
5
Diagnostic step 5
Consider adding a post-restore verification step in the automation that checks for MAC duplicates across the cluster and fails the restore if collisions are detected.