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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
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 3-22. 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
the next two relay agent hops (“B” and “C”). The server can then enforce an
IP addressing policy based on the Option 82 field generated by the edge relay
agent (“A”). In this example, the DHCP policy boundary is at relay agent 1.
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