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.
