Dynamic Host Configuration Protocol (DHCP) is a protocol for assigning dynamic IP addresses to hosts on a local network. DHCP also delivers TCP/IP configuration parameters such as the subnet mask and the default router. DHCP simplifies network administration because the DHCP server keeps track of IP addresses rather than requiring an administrator to manage the task manually.
For DHCP to operate correctly, all host computers need to contact the DHCP server. Routers by default do not forward host broadcasts beyond their own subnet. A DHCP relay agent forwards DHCP packets over routers that do not support the forwarding of these types of packets. A DHCP relay agent is a type of routing protocol that enables DHCP hosts to obtain IP addresses from a DHCP server on a different subnet.
A DHCP relay agent is usually run on a router. The Cisco Catalyst 4500 series switches have routing capabilities. Therefore a Cisco Catalyst 4500 is the appropriate place to run a DHCP relay agent.
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Log in to the Cisco device using an interactive text-oriented communications facility via a virtual terminal application, such as SSH or Telnet. When a login has succeeded, a read-only command-line prompt appears, such as "Switch>."
Type the command "enable" at the "Switch>" prompt. Type the password into the switch. The command-line changes to a enabled prompt, such as "Switch#."
Type the command "config terminal" at the "Switch#" prompt, and press the "Return" key. The "config terminal" command modifies the prompt to "Router (config) #."
Type the command "interface FastEthernet <slot/port>" at the "Router(config)#" prompt, and press the "Return" key. This command modifies the prompt to "Router(config-if)#."
Type the command "ip helper-address <IP address>" at the "Router(config-if)#" prompt. This command enables the forwarding of DHCP packets to a different subnet.
Hold down the "Control" and "Z" keys simultaneously at the "Router(config-if)#" prompt. This takes the router back to the "Router#" prompt. The DHCP relay agent is now configured to support forwarding of DHCP packets.
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