Cisco Cisco Email Security Appliance C160 Mode D'Emploi

You can connect two Ethernet interfaces to the same physical network, but you 
must construct IP addresses and netmasks to allow the Cisco IronPort appliance 
to select a unique delivery interface.

When selecting an interface on which to perform a command or function in the 
GUI or CLI that allows you to select an interface (for example, upgrading 
AsyncOS, or configuring DNS, etc.), routing (your default gateway) will take 
precedence over your selection.

For example, suppose you have an Cisco IronPort appliance with the 3 network 
interfaces configured, each on a different network segment (assume all /24):

And your Default gateway is 192.19.0.1.

Now, if you perform an AsyncOS upgrade (or other command or function that 
allows you to select an interface) and you select the IP that is on data1 
(192.19.1.100), you would expect all the TCP traffic to occur over the data1 
ethernet interface. However, what happens is that the traffic will go out of the 
interface that is set as your default gateway, in this case Management, but will be 
stamped with the source address of the IP on data1.

192.19.0.100

192.19.1.100

192.19.2.100