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IP Routing Features

Configuring DHCP Relay

VLAN

DHCP

Option

Server

Client

DROP

VLAN

APPEND

Relay Agent “A”

Relay Agent “B”

Relay Agent “C”

Figure 5-51. 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

approach to keep a more detailed control over leased IP addresses. In this

example, the primary DHCP policy boundary is at relay agent “A”, but more

global policy boundaries can exist at relay agents “B” and “C”.

Figure 5-52. Example Allowing Only an Upstream Relay Agent To Contribute an Option 82 Field

VLAN

DHCP

Option

Server

Client

DROP

VLAN

No Option 82

REPLACE

Relay Agent “A”

Relay Agent “B”

Relay Agent “C”

Like the first example, above, this configuration drops client requests with

spurious Option 82 fields from clients on the edge relay agent. However, in

this case, only the Option 82 field from the last relay agent is retained for use

by the DHCP server. In this case the DHCP policy boundary is at relay agent

“C”. In the previous two examples the boundary was with relay “A”.

Validation of Server Response Packets

A valid Option 82 server response to a client request packet includes a copy

of the Option 82 field(s) the server received with the request. With validation

disabled, most variations of Option 82 information are allowed, and the

corresponding server response packets are forwarded.

Server response validation is an option you can specify when configuring

Option 82 DHCP for append, replace, or drop operation. (Refer to “Forwarding

Policies” on page 5-127.) Enabling validation on the routing switch can

enhance protection against DHCP server responses that are either from

untrusted sources or are carrying invalid Option 82 information.

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