What happens to vSphere Distributed Switch if vCenter fails ? | VMware

                         What's the biggest question\confusion\concern about VMware vSphere Distributed Switch (vDS)? Yes, you guessed it right… Will the vDS work if my vCenter goes down ?

The answer is yes, it will work (*conditions apply). But how ? How will vDS work , since it depends purely on vCenter. No better time than this to learn about the vDS backend operation.

vDS consists of two parts:

• Control Plane
• Data Plane.

The Control Plane lies in the vCenter Server and Data Plane lies in the ESXi host.

If you browse to the datastores connected to the ESXi host, you can see a folder called .dvsData.

If you open this folder, you will see another folder which has a unique identifier. This is nothing but the identifier of the dvSwitch that is created. This folder has the port information of the virtual machines which have their working directory on this particular datastore. This port info is used by vSphere HA when it restarts a virtual machine which uses a dvPort. (So this also concludes that vSphere HA will work for vDS network without vCenter)

This folder stores the vDS data in addition to the hidden file .dvsdata.db which is stored in /etc/vmware of each ESXi host.

This db is used by the ESXi hosts to connect to the vDS locally during the boot time.

You can see the contents of the vDS database using the below command :

net-dvs -f /etc/vmware/dvsdata.db

Coming back to the real question of this blog post, VMs connectivity during vCenter failure depends on the port binding mechanism chosen during port group creation.

• Static binding - The ports assigned will be static and will not change until the VMs are deleted. This assignment is done during the provisioning time.
• Dynamic binding - The ports assigned will be dynamic and will change if the VMs are restarted or disconnected from network.
• Ephemeral binding - This is similar to a dynamic binding. But if the vCenter is unavailable, administrators will still have the ability to connect directly to an ESXi host and reconfigure VM networking.

If the vCenter goes down and if you need to start a new VM, the port binding should be Static. If it is dynamic, the ports cannot be assigned as vCenter assigns ports. If it is ephemeral, a temporary port is created on the host for the vNIC and the VMs can be powered on. This temporary port will sync up with the vDS once vCenter is back online.

The bottom line is that there should not be any problem if Static Binding is set and even when vCenter is down a VM which has been provisioned before vCenter went down can easily be powered on and will have network access. Ephemeral binding is not used widely as it is mainly meant as a mechanism to ensure that the Portgroup can be chosen from ESXi hosts in case of vCenter failures.