Tunnel Status is "Not Available" for multiple Transport Nodes

Products

• VMware NSX

Issue/Introduction

• Not able to view the tunnel status on the Transport Nodes. It was displaying some tunnels earlier, but currently showing "Not Available" and keep on changing on Other Transport Nodes as well.
  
  
  
• 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

• Since DRS is enabled on the cluster, when overlay segment workloads migrate to other hosts and no overlay segment workloads remain on the affected host, the tunnel status changes to "Not available".

Resolution

• This is expected behaviour because in an overlay-backed segment, traffic between two VMs on different host but attached to the same overlay segment have their layer 2 traffic carried by a tunnel between the hosts, whereas a VLAN-backed segment is a layer 2 broadcast domain that is implemented as a traditional VLAN in the physical infrastructure. This means that traffic between the two VMs on different hosts but attached to the same VLAN-backed segment is carries over a VLAN between the hosts.  

Additional Information

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