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IP Routing Features
Configuring DHCP Relay
Forwarding Policies
DHCP Option 82 on ProCurve switches offers four forwarding policies, with
an optional validation of server responses for three of the policy types (append,
replace, or drop).
Configuration Options for Managing DHCP Client Request
Packets
Option 82
Configuration
DHCP Client Request Packet Inbound to the Routing Switch
Packet Has No
Option 82 Field
Packet Includes an Option 82 Field
Append
Append an
Option 82 Field
Append allows the most detail in defining DHCP policy boundaries. For example,
where the path from a client to the DHCP Option 82 server includes multiple relay
agents with Option 82 capability, each relay agent can define a DHCP policy
boundary and append its own Option 82 field to the client request packet. The server
can then determine in detail the agent hops the packet took, and can be configured
with a policy appropriate for any policy boundary on the path.
Note: In networks with multiple relay agents between a client and an Option 82
server, append can be used only if the server supports multiple Option 82 fields in
a client request. If the server supports only one Option 82 field in a request, consider
using the keep option.
Keep
Append an
Option 82 Field
If the relay agent receives a client request that already has one or more Option 82
fields, keep causes the relay agent to retain such fields and forward the request
without adding another Option 82 field. But if the incoming client request does not
already have any Option 82 fields, the relay agent appends an Option 82 field before
forwarding the request. Some applications for keep include:
• The DHCP server does not support multiple Option 82 packets in a client request
and there are multiple Option 82 relay agents in the path to the server.
• The unusual case where DHCP clients in the network add their own Option 82
fields to their request packets and you do not want any additional fields added
by relay agents.
This policy does not include the validate option (described in the next section) and
allows forwarding of all server response packets arriving inbound on the routing
switch (except those without a primary relay agent identifier.)
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.
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