HP 3500YL - Manual (Page 305)

IP Routing Features
Configuring DHCP Relay
Configuring an IP Helper Address
To add the IP address of a DHCP server for a specified VLAN on a routing
switch, enter the
ip helper-address command at the VLAN configuration level
as in the following example:
ProCurve(config)# vlan 1
ProCurve(vlan-1)# ip helper-address <ip-addr>
To remove the DHCP server helper address, enter the no form of the
command:
ProCurve(vlan-1)# no ip helper-address < ip-addr >
Operating Notes
■ You can configure up to 4000 IP helper addresses on a routing switch. The
helper addresses are shared between the DHCP relay agent and UDP
forwarder feature.
■ A maximum of sixteen IP helper addresses is supported in each VLAN.
Hop Count in DHCP Requests
When a DHCP client broadcasts requests, the DHCP relay agent in the routing
switch receives the packets and forwards them to the DHCP server (on a
different subnet, if necessary). During this process the DHCP relay agent
increments the hop count before forwarding DHCP packets to the server. The
DHCP server, in turn, includes the hop count from the received DHCP request
in the response sent back to a DHCP client.
As a result, the DHCP client receives a non-zero hop count in the DHCP
response packet. Because some legacy DHCP/BootP clients discard DHCP
responses which contain a hop count greater than one, they may fail to boot
up properly. Although this behavior is in compliance with RFC 1542, it
prevents a legacy DHCP/BootP client from being automatically configured
with a network IP address.
Disabling the Hop Count in DHCP Requests
To disable the default behavior of a DHCP relay agent so that the hop count
in a DHCP client request is not increased by one at each hop when it is
forwarded to a DHCP server, enter the
no dhcp-relay hop-count-increment
command at the global configuration level:
ProCurve(config)# no dhcp-relay hop-count-increment
5-131