z/VM Virtual Networking Hints and Tips - NIC Re-creation

• The problem

  For a virtual NIC coupled to an ETHERNET VSWITCH, a LOGOFF of the guest followed quickly by LOGON can result in the inability to establish connectivity to the virtual machine from external hosts if a consistent MAC ID has not been assigned to the NIC. This problem may also occur when a DETACH NIC command is followed by a DEFINE NIC for the same vdev.

  A MAC ID is assigned out of the pool of system MACIDs when a NIC is created by the DEFINE NIC command, or when a NIC is created with a NICDEF statement in the VM directory but no MACID option is specified on the NICDEF. LOGOFF/LOGON or DETACH NIC/DEFINE NIC may result in a new NIC with a different MAC address for the virtual machine.

  Based on the ARP cache time-out values of hosts and switches on the same LAN segment, the MAC address of the original NIC will still be known and used to communicate with the target virtual machine. The previous MAC address will no longer be recognized by the virtual switch. Any communication sent by the virtual machine to an external host will cause the target host ARP cache to be updated.

• The solution

  To prevent these temporary losses of connectivity due to a LOGOFF or a DETACH NIC, configure the MACID option on the NICDEF statement in the guest's directory entry to specify the MAC ID portion of the MAC address. That will make sure that a consistent MAC ID is assigned to the virtual machine.

See the VMLAN MACPREFIX and VMLAN MACIDRANGE CP Configuration statement descriptions for more information about specifying the MAC prefix and SYSTEM and USER MAC ID ranges.

This document is maintained by:

Montana Lee
z/VM Development
montanalee@ibm.com