3-22
Cisco ASA Series Firewall CLI Configuration Guide
Chapter3 Informatio n About NAT
NAT for VPN

Determining the Egress Interface

When the ASA receives traffic for a mapped address, the ASA unstranslates the destination address
according to the NAT rule, and then it sends the packet on to the real address. The ASA determines the
egress interface for the packet in the following ways:
•Transparent mode—The ASA determines the egress interface for the real address by using the NAT
rule; you must specify the source and destination interfaces as part of the NAT rule.
•Routed mode—The ASA determines the egress interface in one of the following ways:
–
You configure the interface in the NAT rule—The ASA uses the NAT rule to determine the
egress interface. However, you have the option to always use a route lookup instead. In certain
scenarios, a route lookup override is required; for example, see the “NAT and VPN Management
Access” section on page 3-26.
–
You do not configure the interface in the NAT rule—The ASA uses a route lookup to determine
the egress interface.
Figure 3-16 shows the egress interface selection method in routed mode. In almost all cases, a route
lookup is equivalent to the NAT rule interface, but in some configurations, the two methods might differ.
Figure3-16 Routed Mode Egress Interface Selection
NAT for VPN
•NAT and Remote Access VPN, page3-23
•NAT and Site-to-Site VPN, page3-24
•NAT and VPN Management Access, page3-26
•Troubleshooting NAT and VPN, page3-28
Real: 10.1.1.78
Mapped: 209.165.201.08
Inside
Untranslation
Packet
Eng
Dest. 209.165.201.08
10.1.1.78209.165.201.08 to
NAT rule specifies interface?
NAT rule specifies route lookup?
NoYes
Yes
No
Send packet out Inside interface.
Where to send 10.1.1.78?
Outside
Look up 10.1.1.78 in routing table.
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