Product guide

IP Routing Features

Configuring DHCP Relay

Multiple Option 82 Relay Agents in a Client Request Path

Where the client is one router hop away from the D H CP 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 inc lu de s

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

DHCP

Option

Server

Client

DROP

VLAN

KEEP

Relay Agent “A”

Relay Agent “B”

Relay Agent “C”

Figure 3-8. 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.

VLAN

DHCP

Option

Server

Client

DROP

VLAN

APPEND

Relay Agent “A”

Relay Agent “B”

Relay Agent “C”

Figure 3-9. Example Configured To Allow Multiple Relay Agents To Contribute an Option 82 Field

This is an enhancement of the previous example. In this case, each hop for an

accepted client request adds a new Option 82 field to the request. A DHCP

server capable of using multiple Option 82 fields can be configured to use this

3-32