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11-3
Cisco ASA 5500 Series Configuration Guide using ASDM
Chapter 11 Configuring Multiple Context Mode
Information About Security Contexts
How the ASA Classifies Packets
Each packet that enters the ASA must be classified, so that the ASA can determine to which context to
send a packet. This section includes the following topics:
• Valid Classifier Criteria, page 11-3
• Classification Examples, page 11-4
Note If the destination MAC address is a multicast or broadcast MAC address, the packet is duplicated and
delivered to each context.
Valid Classifier Criteria
This section describes the criteria used by the classifier and includes the following topics:
• Unique Interfaces, page 11-3
• Unique MAC Addresses, page 11-3
• NAT Configuration, page 11-3
Note For management traffic destined for an interface, the interface IP address is used for classification.
The routing table is not used for packet classification.
Unique Interfaces
If only one context is associated with the ingress interface, the ASA classifies the packet into that
context. In transparent firewall mode, unique interfaces for contexts are required, so this method is used
to classify packets at all times.
Unique MAC Addresses
If multiple contexts share an interface, then the classifier uses the interface MAC address. The ASA lets
you assign a different MAC address in each context to the same shared interface. By default, shared
interfaces do not have unique MAC addresses; the interface uses the burned-in MAC address in every
context. An upstream router cannot route directly to a context without unique MAC addresses. You can
set the MAC addresses manually when you configure each interface (see the “Configuring the MAC
Address and MTU” section on page 14-12), or you can automatically generate MAC addresses (see the
“Automatically Assigning MAC Addresses to Context Interfaces” section on page 11-20).
NAT Configuration
If you do not use unique MAC addresses, then the mapped addresses in your NAT configuration are used
to classify packets. We recommend using MAC addresses instead of NAT, so that traffic classification
can occur regardless of the completeness of the NAT configuration.