VM Guest LAN Support

Tips for using VSWITCH

You can think of the VSWITCH as a Guest LAN with one connection to a real external network. Ordinarily a Guest LAN represents an isolated LAN segment. If you wanted connectivity to the external network you would configure one member of the Guest LAN as a router with a connection to a real network device and a second connection to the Guest LAN. That router would forward messages between the Guest LAN and the external network.

The VSWITCH (in the default NONROUTER mode) simply forwards messages between internal hosts and external hosts based on current IP Address registration. As each internal host initializes the virtual NIC interface, the host IP Address is registered on the NIC and passed to the LAN (or VSWITCH). You can use the QUERY LAN ACTIVE, or QUERY VSWITCH ACTIVE command, to see which IP Addresses are currently registered.

When an internal host sends a message to an unknown IP destination, the VSWITCH automatically sends this message out the VSWITCH interface to the external network. If a message enters the VSWITCH interface with an unknown IP destination, the message is discarded. This is the most efficient mode of operation, but requires that every internal host IP Address must be registered on the VSWITCH.

If all of the internal hosts have adapters coupled to the VSWITCH, the IP Addresses are automatically registered. If, instead, some hosts must be accessed indirectly the options are:

  • Configure the intermediate hosts to register IP addresses on the VSWITCH interface on behalf of remote clients. For example, if LINUX1 at 10.4.1.1 intends to forward traffic for LINUX1B at 10.4.1.5 then LINUX1 could register 10.4.1.5 on the VSWITCH on behalf of LINUX1B. NOTE: I do not know if this is a feasible without creating a second subnet for the remote clients.
  • Configure the VSWITCH and a directly connected host as Primary Router. In this mode, the VSWITCH will pass messages to the primary router when the IP Destination is unknown.

Requirements for a VSWITCH

  1. OSA Express in QDIO mode
    You will need at least one OSA interface (three consecutive devices) bound to the VSWITCH if you want connectivity to the external network. The VSWITCH does NOT have to be the Primary Router unless you specifically need it to pass data through a connected host to a remote client.
  2. z/VM TCP/IP stack acting as controller
    You will need at least one z/VM TCP/IP stack configured to act as a VSWITCH controller.

Tips for using the VSWITCH

  1. Define the VSWITCH with an RDEV option to bind this switch to a set of three OSA Express devices. Use the PORTNAME option to set the portname (up to eight uppercase characters). If you share this OSA card with other hosts, you must all use the same PORTNAME value.
  2. Define the VSWITCH with a CONTROLLER option (usually CONTROLLER *) to indicate which stack users may act as controller. To make a z/VM TCP/IP stack eligible for selection as the current controller:
    (1) USER DIRECT entry for the user must include "IUCV *VSWITCH"
    (2) PROFILE TCPIP must include "VSWITCH CONTROLLER ON"
  3. Grant access to the users who will be allowed to couple to your VSWITCH. You can (optionally) restrict specific users to specific VLAN groups. You must grant access BEFORE these users logon and start trying to couple to the VSWITCH.
  4. DO NOT try to configure DEVICE or LINK statements in the controller stack to represent the VSWITCH connection. You must leave the associated devices clear for CP to use.
  5. DO NOT try to attach the VSWITCH RDEV interface to the controller yourself. You must leave the real devices free for CP to attach as part of VSWITCH processing.
  6. If you were using a Guest LAN with one member acting as the router, then the other guests were probably configured to use the router as the gateway. The router would have used an external host as the gateway to the rest of the network. If you replace a Guest LAN with a VSWITCH you should configure the coupled guests to use the external host as the gateway.
  7. If you want your z/VM TCP/IP stack to have a data connection to the VSWITCH you must define a virtual NIC and couple it to the VSWITCH. DO NOT use virtual device addresses that will conflict with the addresses where CP may attach the real OSA devices.

This document is written and maintained by: 

Dennis Musselwhite
z/VM Development - CP Network Simulation
musselwh@us.ibm.com

The information provided, and views expressed on this site are my own and do not represent the IBM Corporation.