Daren Matthews

This is the base setup of a Cisco Adaptive Security Appliance:

Assume that we are assigned a static public IP address 100.100.100.1 from our ISP. Also, the internal LAN network belongs to subnet 192.168.10.0/24. Interface Ethernet0/0 will be connected on the outside (towards the ISP), and Ethernet0/1 will be connected to the Inside LAN switch.

The firewall will be configured to supply IP addresses dynamically (using DHCP) to the internal hosts. All outbound communication (from inside to outside) will be translated using Port Address Translation (PAT) on the outside public interface. Let’s see a snippet of the required configuration steps for this basic scenario:

Step1: Configure a privileged level password (enable password)

By default there is no password for accessing the ASA firewall, so the first step before doing anything else is to configure a privileged level password, which will be needed to allow subsequent access to the appliance. Configure this under Configuration Mode:

ASA5510(config)# enable password mysecretpassword

Step2: Configure the public outside interface

ASA5510(config)# interface Ethernet0/0

ASA5510(config-if)# nameif outside

ASA5510(config-if)# security-level 0

ASA5510(config-if)# ip address 100.100.100.1 255.255.255.252

ASA5510(config-if)# no shut

Step3: Configure the trusted internal interface

ASA5510(config)# interface Ethernet0/1

ASA5510(config-if)# nameif inside

ASA5510(config-if)# security-level 100

ASA5510(config-if)# ip address 192.168.10.1 255.255.255.0

ASA5510(config-if)# no shut

Step 4: Configure PAT on the outside interface

ASA5510(config)# global (outside) 1 interface

ASA5510(config)# nat (inside) 1 0.0.0.0 0.0.0.0

Step 5: Configure Default Route towards the ISP (assume default gateway is 100.100.100.2)

ASA5510(config)# route outside 0.0.0.0 0.0.0.0 100.100.100.2 1

Step 6: Configure the firewall to assign internal IP and DNS address to hosts using DHCP

ASA5510(config)# dhcpd dns 200.200.200.10

ASA5510(config)# dhcpd address 192.168.10.10-192.168.10.200 inside

ASA5510(config)# dhcpd enable inside

The above basic configuration is just the beginning for making the appliance operational. There are many more configuration features that you need to implement to increase the security of your network, such as Static and Dynamic NAT, Access Control Lists to control traffic flow, DMZ zones, VPN etc.