Option 82 Server Support - HP 2920 Series Multicast And Routing Manual

The routing switch can operate as a DHCP relay agent to enable communication 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 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.
NOTE: The routing switch's DHCP relay information (Option 82) feature can be used in networks
where the DHCP servers 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, see the documentation provided with the server application.
Some client applications can append an Option 82 field to their DHCP requests; see 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.
For information on configuring policies on a server running DHCP Option 82, see the documentation
provided for that application.
66 IP Routing Features