IP Routing Features

 

 

Configuring DHCP Relay

 

 

 

 

Option 82

DHCP Client Request Packet Inbound to the Routing Switch

 

Configuration

 

 

 

Packet Has No

Packet Includes an Option 82 Field

 

 

 

 

Option 82 Field

 

 

 

 

 

 

Replace

Append an

Replace replaces any existing Option 82 fields from downstream relay agents (and/

 

 

Option 82 Field

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

Drop causes the routing switch to drop an inbound client request with an Option

 

 

Option 82 Field

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.)

 

 

 

Relay Agent “A”

 

Relay Agent “B”

 

 

Relay Agent “C”

 

 

 

 

VLAN

 

VLAN

 

VLAN

 

VLAN

 

 

VLAN

 

VLAN

 

 

 

 

 

 

 

 

 

 

 

 

DHCP

Client

 

 

 

 

 

 

20

 

 

 

 

30

 

 

 

 

20

 

 

 

 

10

 

 

 

20

 

 

10

 

 

Option

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

82

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

DROP

 

 

 

 

KEEP

 

 

 

KEEP

Server

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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