Dynamic Host Configuration Protocol (DHCP)

          Every computer or device that connects to the Internet or to an IP network needs an IP address. Most users do not have the expertise to configure an IP address, subnet mask, and gateway. In addition, whenever a computer changes its location in the network, it must receive a new address. Somehow, the address assigned to each device and the addresses that are still available must both be tracked. Most companies do no have the time, resources, or staff to devote to managing such configurations. In addition, networks operate with a finite number of IP addresses. It is most efficient for a host to reserve an address only when it is using it.
          Dynamic Host Configuration Protocol (DHCP) enables hosts on an IP network, called DHCP clients, to lease a temporary IP address from a DHCP server. The server can also issue other configurations to the client that help it function on the network (such as the addresses of Domain Name System (DNS) and Windows Internet Naming Service (WINS) servers). This protocol helps reduce administrative overhead on an IP-based network.
         The ProCurve Secure Router can act as a DHCP server for hosts on directly connected subnets. Router interfaces can also act as DHCP clients and receive a dynamic address from a directly connected DHCP server.

          Understanding the basics of DHCP will help you understand and remember how to configure a DHCP pool. If you can track the DHCP process, you will also find it much easier to troubleshoot the router's DHCP activity.
The DHCP request process breaks down into four steps:
1. The client broadcasts a DHCPDISCOVER packet, requesting an IP address and other configurations.
2. The server responds with a DHCPOFFER, which includes an available network address.
3. The client sends a DHCPREQUEST, accepting the offer and requesting the complete configuration from the server.
4. The server responds with a DHCPACK, which includes:
- the agreed-upon network address
- a default gateway
- a lease time
- the address of one or more DNS servers (optional)
- the address of one or more WINS servers (optional)

         Depending on how you configure the ProCurve Secure Router, the router can act as the DHCP server and/or one of its interfaces can act as a DHCP client (however, an interface that acts as a DHCP client cannot also act as a server).

The ProCurve Secure Router as a DHCP Server
          A router that also functions as a DHCP server is particularly useful for a small-to-medium site at which all subnets connect to the WAN router. The ProCurve Secure Router can connect to up to two switches on its Ethernet ports.

          You should configure one DHCP pool for each subnet. For the default gateway, you would specify the IP address of the Ethernet interface through which the router connects to the subnet.
The switches may also connect to several VLANs. In this case, you would configure VLAN support on the Ethernet interfaces. You would then create a DHCP pool for each VLAN.
           A WAN interface can also act as a server for DHCP clients. However, usually the router at the remote site or a DHCP server would act as the remote network's server. On the other hand, when you bridge two remote sites, one router should act as a DHCP server for all clients in the network.

The ProCurve Secure Router as a DHCP Client
          Some service providers require their subscribers to lease a dynamic address from them. In particular, Frame Relay service providers often require their costumers to use DHCP when connecting to their network. Each permanent virtual circuit (PVC) endpoint receives an IP address only when it needs it. This allows the service provider to conserve the limited number of IP addresses it owns. Internet service providers (ISPs) also often require subscribers to receive an IP address and other configurations from them.
         You must configure the interface that connects to such a provider to act as a DHCP client.
          Ethernet interfaces can also be DHCP clients on the connected subnet. Usually, it is a good idea to assign network nodes a static address.
Interfaces on the ProCurve Secure Router that can take a dynamic address are:
- Ethernet interfaces
- Frame Relay subinterfaces
- Asynchronous Transfer Mode (ATM) subinterfaces
- Point-to-Point Protocol(PPP) interfaces (only when brdging traffic)

            Rather than acting as the server for connected DHCP clients, the router can run DHCP relay, which allows hosts on one subnet to receive configurations from a server on a different subnet. The router receives DHCP packets from clients and forwards them to a remote server on behalf of the clients. Similarly, it receives the committed IP addresses from the server and forwards them to the clients.

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