SDN VNETs on Bonded Uplink Disappear After Adding New VNET (Production Cluster - PVE 9.1.2)

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piyush.chavhan

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Jan 30, 2026
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Hello Proxmox team and community,

We are encountering a critical SDN issue on a production Proxmox VE cluster (9.1.2) involving VNETs backed by a Linux bond interface. The behavior appears to be a regression or edge-case bug triggered by SDN changes on an existing setup.

Environment Overview:
  • Proxmox VE: Production cluster (9.1.2 recent stable release)
  • NW Configs: Phy NICs -> Bond -> Bridge -> SDN -> ZONE -> VNETS
  • SDN enabled
  • Linux bonding (bond interface used as SDN uplink)
  • Multiple existing VNETs mapped to the same bridge (Approx 90 VNETs)
  • VMs actively running on these VNETs
Timeline & Issue Description:
  • The bond interface was correctly detected, stable, and fully functional.
  • SDN VNETs were already associated with the bond, and:
    • VNETs were visible in the UI
    • VM connectivity was stable
    • No packet loss or SDN-related issues were observed
  • Recently, we added an additional VNET to the same existing SDN zone / bond uplink
  • Immediately after this change:
    • VNETs associated with the bond disappeared
    • VMs attached to the affected VNETs lost network connectivity and went down
    • SDN configuration became inconsistent/unusable
  • Rolling back or restarting services did not restore connectivity
Key Observations:
  • This is not an initial setup issue — the configuration was working correctly for a significant period
  • The failure was triggered by adding a new VNET to an already working bonded SDN configuration
  • Impact was immediate and affected running production workloads
  • Behavior suggests a state inconsistency or SDN recalculation issue when modifying VNETs on bonded uplinks.
WORKAROUND:

As a temporary mitigation, the issue was resolved using the following steps:
  1. Performed an apt upgrade to bring Proxmox VE and related SDN packages to the latest available versions (9.1.4).
  2. After the upgrade, re-applied the SDN configuration via the Proxmox UI (without changing the existing design).
  3. Once the SDN configuration was re-applied:
    • VNETs reappeared correctly
    • VM network connectivity was restored
    • Bonded uplink functioned as expected
Questions for the Community / Proxmox Team
  1. Is this a known SDN bug related to bonded uplinks, especially when modifying existing VNET configurations? ( PS: I did search the bug list for 9.1.2 could'nt find any)
  2. Has this issue been addressed or fixed in the latest Proxmox VE releases?
  3. Are there specific bonding modes (e.g., active-backup vs LACP) recommended or required for SDN?
  4. Are there any mandatory configuration steps or limitations when:
    • Adding new VNETs
    • Modifying SDN objects
    • Using bonds as SDN uplinks in production
  5. Is there a safe procedure for extending SDN VNETs on bonded interfaces without disrupting live workloads?
Given the production impact, we’d like to understand whether:
  • This is an SDN design limitation.
  • A known bug.
  • Or a configuration requirement that needs to be adjusted.
Any insights, confirmation, or guidance on best practices for SDN + bond in production environments would be greatly appreciated.

Thank you for your time and support.
 
piyush.chavhan said:
Questions for the Community / Proxmox Team
  1. Is this a known SDN bug related to bonded uplinks, especially when modifying existing VNET configurations? ( PS: I did search the bug list for 9.1.2 could'nt find any)
  2. Has this issue been addressed or fixed in the latest Proxmox VE releases?
  3. Are there specific bonding modes (e.g., active-backup vs LACP) recommended or required for SDN?
  4. Are there any mandatory configuration steps or limitationswhen:
    • Adding new VNETs
    • Modifying SDN objects
    • Using bonds as SDN uplinks in production
  5. Is there a safe procedure for extending SDN VNETs on bonded interfaces without disrupting live workloads?
Click to expand...
If this is a production environment, using the probabely subscribed Proxmox Support instead of the forum to get the verified responses, especially for your questions 1+2, might be the better option.

3. no, as the bonding modes are not relevant for the SDN layer
4. not that I have heard of.
5. same question like 4

For deeper analysis configuration files and logs of the encountered situation would be helpful.

PS: It seems like a chatbot was used to aid in creating your post. Would be very helpul to tell it to shorten the post to relevant parts, without repeating same information again and again.
 
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