HP 3500YL - Manual (Page 312)

5-138
IP Routing Features
Configuring DHCP Relay
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.
Option 82 Field Content
The Remote ID and Circuit ID subfields comprise the Option 82 field a relay
agent appends to client requests. A DHCP server configured to apply a
different IP addressing policy to different areas of a network uses the values
in these subfields to determine which DHCP policy to apply to a given client
request.
■ Remote ID: This configurable subfield identifies a policy area that
comprises either the routing switch as a whole (by using the routing
switch MAC address) or an individual VLAN configured on the routing
switch (by using the IP address of the VLAN receiving the client request).
• Use the IP address option if the server will apply different IP
addressing policies to DHCP client requests from ports in different
VLANs on the same routing switch.
Figure 5-63. Example of DHCP Option 82 Operation in a Network with a Non-Compliant Relay Agent
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