Recently, I setup a new Cisco ASA for a customer who is stuck using a cable-modem in their office. Their particular ISP assigns a DHCP address to all clients, and makes that IP sticky to the MAC Address it is initially assigned to. If a different MAC address requests an IP through that modem, it is assigned a different address.
Since this customer’s public mail-exchanger is hosted at this address, if the IP changes, e-mail will stop arriving until we resolve the situation. To prevent this I configured the ASA to present the MAC address from the existing router when it made its initial DHCP request to the cable-company.
Continue reading How To: Configure MAC Address on Cisco ASA WAN Interface