Tunnel Status is "Not Available" for single Transport nodes or multiple Transport Nodes

Products

• VMware NSX

Issue/Introduction

• Not able to view the tunnel status on the Transport Nodes. NSX UI was displaying some tunnels earlier, but are currently showing "Not Available" and keep on changing on other Transport Nodes as well.
  
  
  
• In Multi-TEP configurations, some hosts show a full set of tunnels (e.g., 30+), while others show a reduced count (e.g., ~18).
• Status of the Transport node shows Up and in Success State.
• Transport Node is not in "Maintenance Mode" and have multiple Workload VMs as well.
• Verified the controller and Manager connectivity:
  On the Transport Node, we can see that both the Controller and Manager are connected successfully:
  > get managers 
> get controllers 

Environment

• VMware NSX
• NSX-T Data Center

Cause

This is expected behavior. NSX utilizes dynamic tunnel establishment to conserve host and network resources. TEP (Tunnel Endpoint) tunnels are only built and maintained when active overlay workload traffic requires that specific interface or datapath.

• Zero Workloads: If no VMs on overlay segments reside on a host, no TEP tunnels are established.
• Multi-TEP Hashing: In a Multi-TEP setup, if all resident overlay VMs hash to a single physical uplink (e.g., vmnic2), the tunnels associated with the secondary TEP (e.g., on vmnic3) remain idle and spun down until traffic utilizes that uplink.

Resolution

No configuration changes are required as the system functions as designed. Tunnels establish automatically once overlay traffic is detected on the interface.

To verify the datapath and hashing association on an ESXi host, follow these steps:

1. Log in to the ESXi host CLI via SSH.
2. Run the following command to list vSwitch instances and interface mappings: nsxdp-cli vswitch instance list
3. Confirm if overlay VMs associate with only one physical uplink.
4. Trigger tunnel establishment for testing by migrating an overlay VM to the host or deploying a new VM. As workloads distribute across uplinks, the tunnels activate.

Additional Information

Please refer to the document which explain the concept of segments in NSX : VMware NSX 4.2 Documentation/Segments