Persistent Networks
The network that you can provision without having to deploy any instances 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 instance is created on that network. When the first instance is created, a VLAN ID is assigned and the network is provisioned. Also, when the last instance 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 instances. 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 Network Tier consisting of only physical devices. For example, you might create a VPC for a three-tier application, deploy instances 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 instances are destroyed the services will not be discontinued.
Persistent Network Considerations
Persistent network is designed for isolated and L2 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.
Creation of an Isolated Persistent network will deploy a Virtual Router when the network is created.
Creation of an L2 Persistent network setups up the network devices namely, bridges, VLANs or port-groups across all hosts in a zone.
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 instances, 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 instances, during the next network garbage collection run the network is shut down.
When the last instance 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.
Creating a Persistent Guest Network
To create a persistent network, perform the following:
Create a network offering with the Persistent option enabled.
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.
Click OK.