HP 3500YL - Manual (Page 301)

IP Routing Features
Configuring DHCP Relay
Configuring DHCP Relay
Overview
The Dynamic Host Configuration Protocol (DHCP) is used for configuring
hosts with IP address and other configuration parameters without user inter­
vention. The protocol is composed of three components:
■ DHCP client
■ DHCP server
■ DHCP relay agent
The DHCP client sends broadcast request packets to the network; the DHCP
servers respond with broadcast packets that offer IP parameters, such as an
IP address for the client. After the client chooses the IP parameters, commu­
nication between the client and server is by unicast packets.
ProCurve routing switches provide the DHCP relay agent to enable commu­
nication from a DHCP server to DHCP clients on subnets other than the one
the server resides on. The DHCP relay agent transfers DHCP messages from
DHCP clients located on a subnet without a DHCP server to other subnets. It
also relays answers from DHCP servers to DHCP clients.
The DHCP relay agent is transparent to both the client and the server. Neither
side is aware of the communications that pass through the DHCP relay agent.
As DHCP clients broadcast requests, the DHCP relay agent receives the
packets and forwards them to the DHCP server. During this process, the DHCP
relay agent increases the hop count by one before forwarding the DHCP
message to the server. A DHCP server includes the hop count from the DHCP
request that it receives in the response that it returns to the client.
DHCP Packet Forwarding
The DHCP relay agent on the routing switch forwards DHCP client packets to
all DHCP servers that are configured in the table administrated for each VLAN.
Unicast Forwarding
The packets are forwarded using unicast forwarding if the IP address of the
DHCP server is a specific host address. The DHCP relay agent sets the
destination IP address of the packet to the IP address of the DHCP server and
forwards the message.
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