HP 2610-PWR - Manual (Page 286)

IP Routing Features
Configuring DHCP Relay
N o t e
DHCP 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 both 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.
DHCP Option 82 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 broad­
casting 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.
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