When you do not specify a MAC address in creating logical domain or a network device, the Logical Domains Manager automatically allocates and assigns a MAC address to that logical domain or network device. To obtain this MAC address, the Logical Domains Manager iteratively attempts to select an address and then checks for potential collisions.
Before selecting a potential address, the Logical Domains Manager first looks to see if it has a recently freed, automatically assigned address saved in a database for this purpose (see Freed MAC Addresses). If so, the Logical Domains Manager selects its candidate address from the database.
If no recently freed addresses are available, the MAC address is randomly selected from the 256K range of addresses set aside for this purpose. The MAC address is selected randomly to lessen the chance of a duplicate MAC address being selected as a candidate.
The address selected is then checked against other Logical Domains Managers on other systems to prevent duplicate MAC addresses from actually being assigned. The algorithm employed is described in Duplicate MAC Address Detection. If the address is already assigned, the Logical Domains Manager iterates, choosing another address, and again checking for collisions. This continues until a MAC address is found that is not already allocated, or a time limit of 30 seconds has elapsed. If the time limit is reached, then the creation of the device fails, and an error message similar to the following is shown.
Automatic MAC allocation failed. Please set the vnet MAC address manually.