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Etherstubs seem to confuse some people. Things usually get cleared up when I compare the to vSwitches.
Take the following example:
Say you run each component in either zone or KVM instance, you can then give each a vnic that is linked to your main network interface. But will put more traffic on your physical switch and also open up some security issues. You could ofcourse use VLAN's but that again adds more administrative complexity.
To solve this you can use an etherstub with vnics for the database and application servers. The proxy will also have a vnic on the etherstub and one on the physical interface.
This approuce will:
Bellow I'll give some examples on how to create an etherstub and how to add vnics to it.