HP 2610-PWR - Manual (Page 293)

IP Routing Features
Configuring DHCP Relay
Option 82
Configuration DHCP Client Request Packet Inbound to the Routing Switch
Packet Has No
Option 82 Field Packet Includes an Option 82 Field
Replace Append an
Option 82 Field Replace replaces any existing Option 82 fields from downstream relay agents (and/
or the originating client) with an Option 82 field for the current relay agent.. Some
applications for replace include:
• The relay agent is located at a point in the network that is a DHCP policy
boundary and you want to replace any Option 82 fields appended by down­
stream devices with an Option 82 field from the relay agent at the boundary.
(This eliminates downstream Option 82 fields you do not want the server to use
when determining which IP addressing policy to apply to a client request.)
• In applications where the routing switch is the primary relay agent for clients
that may append their own Option 82 field, you can use replace to delete these
fields if you do not want them included in client requests reaching the server.
Drop Append an
Option 82 Field Drop causes the routing switch to drop an inbound client request with an Option
82 field already appended. If no Option 82 fields are present, drop causes the routing
switch to add an Option 82 field and forward the request. As a general guideline,
configure drop on relay agents at the edge of a network, where an inbound client
request with an appended Option 82 field may be unauthorized, a security risk, or
for some other reason, should not be allowed.
Multiple Option 82 Relay Agents in a Client Request Path
Where the client is one router hop away from the DHCP server, only the Option
82 field from the first (and only) relay agent is used to determine the policy
boundary for the server response. Where there are multiple Option 82 router
hops between the client and the server, you can use different configuration
options on different relay agents to achieve the results you want. This includes
configuring the relay agents so that the client request arrives at the server with
either one Option 82 field or multiple fields. (Using multiple Option 82 fields
assumes that the server supports multiple fields and is configured to assign
IP addressing policies based on the content of multiple fields.)
VLAN
10
DHCP
Option
82
Server
Client
DROP
VLAN
20
VLAN
20
VLAN
30
VLAN
10
VLAN
20
KEEP KEEP
Relay Agent “A” Relay Agent “B” Relay Agent “C”
Figure 7-14. Example Configured To Allow Only the Primary Relay Agent To Contribute an Option 82 Field
The above combination allows for detection and dropping of client requests
with spurious Option 82 fields. If none are found, then the drop policy on the
first relay agent adds an Option 82 field, which is then kept unchanged over
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