HP ProCurve 6200yl Multicast And Routing Manual page 238

IP Routing Features
Configuring DHCP Relay
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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
addressing it will return to the client through the usual DHCP server response
packet. This operation provides several advantages over DHCP without
Option 82:
An Option 82 DHCP server can use a relay agent's identity and client
■
source port information to administer IP addressing policies based on
client and relay agent location within the network, regardless of whether
the relay agent is the client's primary relay agent or a secondary agent.
■ A routing switch operating as a primary Option 82 relay agent for DHCP
clients requesting an IP address can enhance network access protection
by blocking attempts to use an invalid Option 82 field to imitate an
authorized client, or by blocking attempts to use response packets with
missing or invalid Option 82 suboptions to imitate valid response packets
from an authorized DHCP server.
An Option 82 relay agent can also eliminate unnecessary broadcast traffic
■
by forwarding an Option 82 DHCP server response only to the port on
which the requesting client is connected, instead of broadcasting the
DHCP response to all ports on the VLAN.
The routing switch's DHCP Relay Information (Option 82) feature can be used
in networks where the DHCP server(s) are compliant with RFC 3046 Option
82 operation. DHCP Servers that are not compliant with Option 82 operation
ignore Option 82 fields. For information on configuring an Option 82 DHCP
server, refer to the documentation provided with the server application.
Some client applications can append an Option 82 field to their DHCP
requests. Refer to the documentation provided for your client application.
It is not necessary for all relay agents on the path between a DHCP client and
the server to support Option 82, and a relay agent without Option 82 should
forward DHCP packets regardless of whether they include Option 82 fields.
However, Option 82 relay agents should be positioned at the DHCP policy
boundaries in a network to provide maximum support and security for the IP
addressing policies configured in the server.