HP 445946-001 - Manual (Page 112)

Basic IP routing
112
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).
The 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 HP 10GbE switches 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 switch 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. The servers respond as a UDP Unicast message back to the
switch, with the default gateway and IP address for the client. The destination IP address in the server
response represents the interface address on the switch that received the client request. This interface
address tells the switch on which VLAN to send the server response to the client.