Just as a real-world bridge connects two sides of a river, a virtual bridge connects a Proxmox virtual network to a physical network. A virtual bridge is like a physical network switch where all virtual machines connect to and can be configured using the Spanning Tree Protocol (STP). A virtual bridge is a great way to create separate subnets. All VMs in the same subnet can connect to their respective bridges. Proxmox creates one virtual bridge by default during the installation process. Each Proxmox node can support up to 4,094 bridges. When the same bridge configuration is entered on all nodes, the bridge can be used from any nodes in the cluster, thus making live migration possible without network connectivity interruption. The default naming format of a bridge is vmbrX, where X represents an integer between 0 to 4,094.

Proxmox will allow a bridge to be created and not be connected to a physical NIC. This allows an isolated environment, that has no access to the physical or any other network on the LAN. Using Open vSwitch, however, we can configure one bridge with multiple VLANs, such as a real physical switch. We will take a look at the Open vSwitch implementation later in this chapter.

We can change a virtual bridge of a VM in real time without needing to power-cycle it. For example, if a VM is configured with a virtual bridge, vmbr0, and we want to change the bridge to vmbr10 later, we can do so without turning off the VM.