HP 3500YL - Manual (Page 308)

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IP Routing Features
Configuring DHCP Relay
Displaying the Hop Count Setting
To verify the current setting for increasing the hop count in DHCP requests,
enter the
show dhcp-relay command. Note that the current setting is displayed
next to DHCP Request Hop Count Increment.
ProCurve# show dhcp-relay
Status and Counters - DHCP Relay Agent
DHCP Relay Agent Enabled : Yes
DHCP Request Hop Count Increment: Disabled
Option 82 Handle Policy : Replace
Remote ID : MAC Address
Client Requests Server Responses
Valid Dropped Valid Dropped
1425 2 1425 0
DHCP Option 82
Option 82 is called the Relay Agent Information option and is inserted by the
DHCP relay agent when forwarding client-originated DHCP packets to a
DHCP server. Servers recognizing the Relay Agent Information option may
use the information to implement IP address or other parameter assignment
policies. The DHCP Server echoes the option back verbatim to the relay agent
in server-to-client replies, and the relay agent strips the option before
forwarding the reply to the client.
The “Relay Agent Information” option is organized as a single DHCP option
that contains one or more “sub-options” that convey information known by
the relay agent. The initial sub-options are defined for a relay agent that is co­
located in a public circuit access unit. These include a “circuit ID” for the
incoming circuit, and a “remote ID” which provides a trusted identifier for the
remote high-speed modem.
The routing switch can operate as a DHCP relay agent to enable communica­
tion between a client and a DHCP server on a different subnet. Without Option
82, DHCP operation modifies client IP address request packets to the extent
needed to forward the packets to a DHCP server. Option 82 enhances this
operation by enabling the routing switch to append an Option 82 field to such
client requests. This field includes two suboptions for identifying the routing
switch (by MAC address or IP address) and the routing switch port the client
is using to access the network. A DHCP server with Option 82 capability can
read the appended field and use this data as criteria for selecting the IP
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