In multiple context mode, auto-generation assigns unique MAC addresses to all interfaces assigned to a context.
In single context mode, auto-generation assigns unique MAC addresses to VLAN subinterfaces.
If you manually assign a MAC address and also enable auto-generation, then the manually assigned MAC address is used. If you later remove the manual MAC address, the auto-generated address is used, if enabled.
In the rare circumstance that the generated MAC address conflicts with another private MAC address in your network, you can manually set the MAC address for the interface.
Because auto-generated addresses (when using a prefix) start with A2, you cannot start manual MAC addresses with A2 if you also want to use auto-generation.
The ASA generates the MAC address using the following format:
Where xx.yy is a user-defined prefix or an autogenerated prefix based on the last two bytes of the interface MAC address, and zz.zzzz is an internal counter generated by the ASA. For the standby MAC address, the address is identical except that the internal counter is increased by 1.
For an example of how the prefix is used, if you set a prefix of 77, then the ASA converts 77 into the hexadecimal value 004D (yyxx). When used in the MAC address, the prefix is reversed (xxyy) to match the ASA native form:
For a prefix of 1009 (03F1), the MAC address is:
The MAC address format without a prefix is a legacy version. See the mac-address auto command in the command reference for more information about the legacy format.