General Dhcp Option 82 Requirements And Operation; Requirements; General Dhcp-Relay Operation With Option 82 - HP 2920 Series Multicast And Routing Manual

Figure 5 Example of a DHCP Option 82 application

General DHCP Option 82 requirements and operation

Requirements

DHCP Option 82 operation is configured at the global config level and requires the following:
IP routing enabled on the switch
DHCP-relay option 82 enabled (global command level)
Routing switch access to an Option 82 DHCP server on a different subnet than the clients
requesting DHCP Option 82 support
One IP helper address configured on each VLAN supporting DHCP clients

General DHCP-relay operation with Option 82

Typically, the first (primary) Option 82 relay agent to receive a client's DHCP request packet
appends an Option 82 field to the packet and forwards it toward the DHCP server identified by
the IP helper address configured on the VLAN in which the client packet was received. Other,
upstream relay agents used to forward the packet may append their own Option 82 fields, replace
the Option 82 fields they find in the packet, forward the packet without adding another field, or
drop the packet. (Intermediate next-hop routing switches without Option 82 capability can be used
to forward—route—client request packets with Option 82 fields.) Response packets from an Option
82 server are routed back to the primary relay agent (routing switch) and include an IP addressing
assignment for the requesting client and an exact copy of the Option 82 data the server received
with the client request. The relay agent strips off the Option 82 data and forwards the response
packet out the port indicated in the response as the Circuit ID (client access port.) Under certain
validation conditions described later in this section, a relay agent detecting invalid Option 82 data
in a response packet may drop the packet.
Configuring DHCP relay 67