BLADE OS™ Application Guide HP GbE2c Ethernet Blade Switch for c-Class BladeSystem Version 5.1 Advanced Functionality Software

BLADE OS 5.1 Application Guide
164  Chapter 9: Basic IP Routing BMD00113, September 2009
Dynamic Host Configuration Protocol
Dynamic Host Configuration Protocol (DHCP) is a transport protocol that provides a framework for
automatically assigning IP addresses and configuration information to other IP hosts or clients in a
large TCP/IP network. Without DHCP, the IP address must be entered manually for each network
device. DHCP allows a network administrator to distribute IP addresses from a central point and
automatically send a new IP address when a device is connected to a different place in the network.
DHCP is an extension of another network IP management protocol, Bootstrap Protocol (BOOTP),
with an additional capability of being able to dynamically allocate reusable network addresses and
configuration parameters for client operation.
Built on the client/server model, DHCP allows hosts or clients on an IP network to obtain their
configurations from a DHCP server, thereby reducing network administration. The most significant
configuration the client receives from the server is its required IP address; (other optional parameters
include the “generic” file name to be booted, the address of the default gateway, and so forth).
DHCP relay agent eliminates the need to have DHCP/BOOTP servers on every subnet. It allows the
administrator to reduce the number of DHCP servers deployed on the network and to centralize
them. Without the DHCP relay agent, there must be at least one DHCP server deployed at each
subnet that has hosts needing to perform the DHCP request.
DHCP Relay Agent
DHCP is described in RFC 2131, and the DHCP relay agent supported on the GbE2c is described in
RFC 1542. DHCP uses UDP as its transport protocol. The client sends messages to the server on
port 67 and the server sends messages to the client on port 68.
DHCP defines the methods through which clients can be assigned an IP address for a finite lease
period and allowing reassignment of the IP address to another client later. Additionally, DHCP
provides the mechanism for a client to gather other IP configuration parameters it needs to operate
in the TCP/IP network.
In the DHCP environment, the GbE2c acts as a relay agent. The DHCP relay feature
(/cfg/l3/bootp) enables the switch to forward a client request for an IP address to two BOOTP
servers with IP addresses that have been configured on the switch.
When a switch receives a UDP broadcast on port 67 from a DHCP client requesting an IP address,
the switch acts as a proxy for the client, replacing the client source IP (SIP) and destination IP (DIP)
addresses. The request is then forwarded as a UDP Unicast MAC layer message to two BOOTP
servers whose IP addresses are configured on the switch. Each server respond to the switch using a
UDP Unicast message, with the default gateway and IP address for the client. The destination IP
address in the server response represents the switch interface address that received the client request.
This interface address tells the switch on which VLAN to send the server response to the client.