HP 2610-PWR - Manual (Page 289)

IP Routing Features
Configuring DHCP Relay
■ 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
field(s) 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.
Switch
Client
DHCP
Option 82
Server
Relay Agent 1 adds an Option 82 field to a
client request, and then forwards the
request toward the server. This includes
any client requests received from Relay
Agent 2 without an Option 82 field.
Note: DHCP Option 82 does not operate
with clients on VLAN 4 because DHCP
requests from these clients are not routed.
Relay Agent 2 does not add an Option 82
field to client requests before forwarding
the requests. However, any client
requests received from Relay Agent 3 will
be forwarded with the Option 82 fields that
were added by Relay Agent 3.
Relay Agent 3 adds an Option 82 field to a
client request and then forwards the
request.
Client
Client
Switch
Relay Agent “2”
VLAN 1 VLAN 2
VLAN 3 VLAN 2
VLAN 3 VLAN 4
Switch
Client
Switch
Client
Switch
Client Client
Relay Agent “1”
Relay Agent “3”
No Option 82
Option 82 Enabled
Option 82 Enabled
Client
Figure 7-11. Example of DHCP Option 82 Operation in a Network with a Non-Compliant Relay Agent
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