The network that you can provision without having to deploy any VMs on it is called a persistent network. A persistent network can be part of a VPC or a non-VPC environment.
When you create other types of network, a network is only a database entry until the first VM is created on that network. When the first VM is created, a VLAN ID is assigned and the network is provisioned. Also, when the last VM is destroyed, the VLAN ID is released and the network is no longer available. With the addition of persistent network, you will have the ability to create a network in CloudStack in which physical devices can be deployed without having to run any VMs. Additionally, you can deploy physical devices on that network.
One of the advantages of having a persistent network is that you can create a VPC with a tier consisting of only physical devices. For example, you might create a VPC for a three-tier application, deploy VMs for Web and Application tier, and use physical machines for the Database tier. Another use case is that if you are providing services by using physical hardware, you can define the network as persistent and therefore even if all its VMs are destroyed the services will not be discontinued.
13.20.1. Persistent Network Considerations
Persistent network is designed for isolated networks.
All default network offerings are non-persistent.
A network offering cannot be editable because changing it affects the behavior of the existing networks that were created using this network offering.
When you create a guest network, the network offering that you select defines the network persistence. This in turn depends on whether persistent network is enabled in the selected network offering.
An existing network can be made persistent by changing its network offering to an offering that has the Persistent option enabled. While setting this property, even if the network has no running VMs, the network is provisioned.
An existing network can be made non-persistent by changing its network offering to an offering that has the Persistent option disabled. If the network has no running VMs, during the next network garbage collection run the network is shut down.
When the last VM on a network is destroyed, the network garbage collector checks if the network offering associated with the network is persistent, and shuts down the network only if it is non-persistent.
13.20.2. Creating a Persistent Guest Network
To create a persistent network, perform the following:
Create a network offering with the Persistent option enabled.
See the Administration Guide.
Select Network from the left navigation pane.
Select the guest network that you want to offer this network service to.
Click the Edit button.
From the Network Offering drop-down, select the persistent network offering you have just created.
