Firewall Functional Overview - Cisco ASA Series Cli Configuration Manual

Chapter 1 Introduction to the Cisco ASA

Firewall Functional Overview

For multiple context mode, if you place the router behind the ASASM, you should only connect it to a
single context. If you connect the router to multiple contexts, the router will route between the contexts,
which might not be your intention. The typical scenario for multiple contexts is to use a router in front
of all the contexts to route between the Internet and the switched networks (see Figure 1-2).
Figure 1-2 MSFC/Router Placement with Multiple Contexts
Internet
VLAN 100
MSFC/Router
VLAN 300 VLAN 303
VLAN 301 VLAN 302
Admin
Context A Context B Context C
Context
VLAN 200 VLAN 201 VLAN 202 VLAN 203
Admin Inside Inside
Inside
Network Customer A Customer C
Customer B
Firewall Functional Overview
Firewalls protect inside networks from unauthorized access by users on an outside network. A firewall
can also protect inside networks from each other, for example, by keeping a human resources network
separate from a user network. If you have network resources that need to be available to an outside user,
such as a web or FTP server, you can place these resources on a separate network behind the firewall,
called a demilitarized zone (DMZ). The firewall allows limited access to the DMZ, but because the DMZ
only includes the public servers, an attack there only affects the servers and does not affect the other
inside networks. You can also control when inside users access outside networks (for example, access to
the Internet), by allowing only certain addresses out, by requiring authentication or authorization, or by
coordinating with an external URL filtering server.
When discussing networks connected to a firewall, the outside network is in front of the firewall, the
inside network is protected and behind the firewall, and a DMZ, while behind the firewall, allows limited
access to outside users. Because the ASA lets you configure many interfaces with varied security
policies, including many inside interfaces, many DMZs, and even many outside interfaces if desired,
these terms are used in a general sense only.
Cisco ASA Series CLI Configuration Guide
1-26