HP 3500YL - Manual (Page 309)

IP Routing Features
Configuring DHCP Relay
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.
N o t e 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.
Option 82 Server Support
To apply DHCP Option 82, the routing switch must operate in conjunction with
a server that supports Option 82. (DHCP servers that do not support Option
82 typically ignore Option 82 fields.) Also, the routing switch applies Option
82 functionality only to client request packets being routed to a DHCP server.
DHCP relay with Option 82 does not apply to switched (non-routed) client
requests.
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