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Cisco ASA Series CLI Configuration Guide
Chapter 1 Configuring Multiple Context Mode
Changing Between Contexts and the System Execution Space
Automatically Assigning MAC Addresses to Context Interfaces
This section describes how to configure auto-generation of MAC addresses.
The MAC address is used to classify packets within a context. See the “Information About MAC
Addresses” section on page 1-11 for more information, especially if you are upgrading from an earlier
ASA version. See also the “Viewing Assigned MAC Addresses” section on page 1-37.
Guidelines
• When you configure a nameif command for the interface in a context, the new MAC address is
generated immediately. If you enable this feature after you configure context interfaces, then MAC
addresses are generated for all interfaces immediately after you enable it. If you disable this feature,
the MAC address for each interface reverts to the default MAC address. For example, subinterfaces
of GigabitEthernet 0/1 revert to using the MAC address of GigabitEthernet 0/1.
• 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 within the context.
See the “Configuring the MAC Address and MTU” section on page 1-10 to manually set the MAC
address.
Detailed Steps
Changing Between Contexts and the System Execution
Space
If you log in to the system execution space (or the admin context), you can change between contexts and
perform configuration and monitoring tasks within each context. The running configuration that you edit
in a configuration mode, or that is used in the copy or write commands, depends on your location. When
you are in the system execution space, the running configuration consists only of the system
configuration; when you are in a context, the running configuration consists only of that context. For
example, you cannot view all running configurations (system plus all contexts) by entering the show
running-config command. Only the current configuration displays.
Command Purpose
mac-address auto [prefix prefix]
Example:
hostname(config)# mac-address auto prefix
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Automatically assigns private MAC addresses to each context interface.
If you do not enter a prefix, then the ASA autogenerates the prefix based
on the last two bytes of the interface (ASA 5500) or backplane (ASASM)
MAC address.
If you manually enter a prefix, then the prefix is a decimal value between
0 and 65535. This prefix is converted to a four-digit hexadecimal number,
and used as part of the MAC address. See the “MAC Address Format”
section on page 1-12 section for more information about how the prefix is
used.
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