The ASA 5500 series firewall can work as DHCP relay agent which means that it receives DHCP requests from clients on one interface and forwards the requests to a DHCP server on another interface. Usually the DHCP server is located in the same layer 3 subnet with its clients. There are situations however where we have only one DHCP server but several layer 3 networks exist (on different security zones on a Cisco ASA) and dynamic IP allocation is required for those networks as well. With the DHCP relay feature, we can connect the DHCP server on one network zone and have the firewall forward all DHCP requests from the other network zones to the DHCP server.

The diagram below illustrates a simple network scenario with three security zones (network interfaces) and a single DHCP server. The three network zones are inside, outside and DMZ. The DHCP clients are connected to the inside network and the DHCP server on the DMZ network. The DHCP requests from the clients on the inside network will be relayed to the server on the DMZ network. The server will assign IP addresses in the range 192.168.1.0/24 to the clients.

Configuration

!First identify the DHCP server and the interface it Is connected to
ciscoasa# conf t
ciscoasa(config)# dhcprelay server 10.1.1.100 DMZ
ciscoasa(config)# dhcprelay timeout 90

!Now enable the DHCP relay on the inside interface
ciscoasa(config)# dhcprelay enable inside

!Assign the ASA inside interface IP as default gateway for the clients
ciscoasa(config)# dhcprelay setroute inside

Usage Guidelines

You can add up to four DHCP relay servers per interface. You must add at least one dhcprelay server command to the ASA Firewall configuration before you can enter the dhcprelay enable command. You cannot configure a DHCP client on an interface that has a DHCP relay server configured.

You cannot enable DHCP relay under the following conditions:
• You cannot enable DHCP relay and the DHCP relay server on the same interface.
• You cannot enable DCHP relay and a DHCP server (dhcpd enable) on the same interface.

This article describes the user interface and access modes and commands associated with the operation of Cisco ASA 5500 firewall appliances. We assume that you know how to connect to the appliance using a console cable (the blue flat cable with RJ-45 on one end, and DB-9 Serial on the other end) and a Terminal Emulation software (e.g HyperTerminal), and how to use basic Command Line Interface.

SECURITY APPLIANCE ACCESS MODES
A Cisco ASA security appliance has four main administrative access modes:

• Monitor Mode: Displays the monitor> prompt. A special mode that enables you to update the image over the network or to perform password recovery. While in the monitor mode, you can enter commands to specify the location of a TFTP server and the location of the software image or password recovery binary image file to download. You access this mode by pressing the “Break” or “ESC” keys immediately after powering up the appliance.
• Unprivileged Mode: Displays the > prompt. Available when you first access the appliance. If the appliance is a Cisco PIX 500 series, the prompt for unprivileged mode is pixfirewall> and if the appliance is the new Cisco ASA 5500 Series, the prompt is ciscoasa>

This mode provides restricted view of the security appliance. You cannot configure anything from this mode. To get started with configuration, the first command you need to know is the enable command. Type enable and hit Enter. The initial password is empty, so hit Enter again to move on the next access mode (Privileged Mode).

• Privileged Mode: Displays the # prompt. Enables you to change the current settings. Any unprivileged command also works in this mode. From this mode you can see the current configuration by using “show running-config”. Still, you cannot configure anything yet until you go to Configuration Mode. You access the Configuration Mode using the configure terminal command from the Privileged Mode.

• Configuration Mode: This mode displays the (config)# prompt. Enables you to change all system configuration settings. Use exit from each mode to return to the previous mode.

The (config)# mode is sometimes called Global Configuration Mode. Some configuration commands from this mode enter a command-specific mode and the prompt changes accordingly. For example the interface command enters interface configuration mode as shown below:

ciscoasa(config)# interface GigabitEthernet0/1
ciscoasa(config-if)# <– Configure Interface specific parameters

Traditionally, a network firewall is a routed hop that acts as a default gateway for hosts that connect to one of its screened subnets. A transparent firewall (or Layer 2 firewall), on the other hand, acts like a “stealth firewall” and is not seen as a Layer 3 hop to connected devices. The appliance connects the same Layer 3 network subnet on its inside and outside ports, but each interface of the firewall resides in a different Layer 2 Vlan. The Cisco ASA firewall can operate both in Routed Firewall Mode (default mode) or in Transparent Firewall Mode.

Routed Firewall Mode:

See the diagram below for a common network topology of a Cisco ASA firewall working in Routed Mode.

As you can see, there are two different network subnets. Inside network (10.20.20.0/24) and Outside Network (10.10.10.0/24). There must be also two different layer2 vlans (Vlan20 for inside network and Vlan10 for outside network). All hosts residing in internal network must belong to subnet 10.20.20.0 and must have default gateway the internal IP of the ASA (10.20.20.1).

Transparent Firewall Mode:

The diagram below shows an example topology using a Cisco ASA in Layer 2 transparent mode.

As you can see, there is only one Layer 3 network (10.10.10.0/24) BUT there MUST be two different Layer 2 Vlans (Vlan20 for inside zone and Vlan10 for outside zone). All hosts must reside in network range 10.10.10.0 and the devices must have as default gateway the IP address of the outside router (10.10.10.2). Also, a management IP address MUST be configured on the ASA firewall (again within the range of 10.10.10.0). DO NOT specify the management IP address of the ASA as the default gateway for connected devices.

Characteristics of Transparent Mode

• Transparent firewall mode supports only two interfaces (inside and outside)
• The firewall bridges packets from one VLAN to the other instead of routing them.
• MAC lookups are performed instead of routing table lookups.
• Can run in single firewall context or in multiple firewall contexts.
• A management IP address is required on the ASA.
• The management IP address must be in the same subnet as the connected network.
• Each interface of the ASA must be a different VLAN interface.
• Even though the appliance acts as a Layer 2 bridge, Layer 3 traffic cannot pass through the security appliance from a lower security level to a higher security level interface.
• The firewall can allow any traffic through by using normal extended Access Control Lists (ACL).

Initial Configuration

Asa(config)# firewall transparent

!Configure management IP below
Asa(config)# ip address 10.10.10.1 255.255.255.0

Asa(config)# interface Ethernet0/0
Asa(config-if)# nameif outside
Asa(config-if)# security-level 0
!
Asa(config)# interface Ethernet0/1
Asa(config-if)# nameif inside
Asa(config-if)# security-level 100