Guest LAN versus Virtual Switch

Depending on your requirements, you may define guest LANs, virtual switches, or a combination for your system. A guest LAN is a closed LAN where each virtual machine can communicate only with the other virtual machines in the same guest LAN. To connect a guest LAN to an external network, you must do one of the following:
  • Use a router virtual machine that has connectivity to both the guest LAN (via a virtual network adapter) and a real hardware network device.
  • Deploy a virtual switch.

A virtual switch is a special-purpose guest LAN that can be defined with connections to one or more real Start of changeOSAEnd of change adapters as its Uplink port. You can also define a virtual switch without the Start of changeOSAEnd of change adapter, in which case the virtual switch acts like a guest LAN. A third choice is to define a virtual switch with the Uplink port directed to a guest port on the virtual switch. With a virtual switch, the requirement for a router virtual machine is eliminated.

VLAN support differs dramatically when deployed on a virtual switch as opposed to on a guest LAN:
  • VLAN support for a guest LAN is a host initiated enablement for the NIC device with no authorization applied during the joining of VLAN LAN segments. This adapter endpoint type of support is virtualized by z/VM® on a virtual NIC device level.
  • VLAN support for a virtual switch device is a representation of a physical networking switch with full authorization on a per port basis for membership in a VLAN LAN segment.

This support represents two different levels of virtualization in z/VM. Either of these may be appropriate based on the intended environment. Even though both levels can be deployed concurrently, this is not recommended due to the added complication in determining topology behavior. For example, if you configure a host to define a particular VLAN ID for a given link, and this host is coupled to a virtual switch that is configured as VLAN-aware, the potential exists that a conflict between the two virtualization layers (NIC and virtual switch) will prevent the guest from establishing connectivity to the desired LAN segment. It is recommended that you do not configure a VLAN ID on the link for a VLAN-aware virtual switch.