ASA 5505 Under the Hood

In part of preparing to run a training on Cisco ASA's for some help desk techs I drew up a diagram to logically show how the ASA 5505 works under the hood in regards to its 8 port switch and virtual interfaces for passing traffic.

Inside the ASA 5505 there is a 8port 10/100 switch that functions just like you would expect any old layer 2 switch to work. Its only through the use of virtual "VLAN interfaces" that the ASA 5505 is able to route, control, and NAT traffic. At its most basic a VLAN interface is virtual interface that exists inside the ASA software and is bound to a layer2 VLAN. It also acts like a routed interface on a ASA 5510 and higher, a normal router or a VLAN interface on a layer 3 switch.