IP Routing Features

Configuring DHCP Relay

DHCP Option 82

DHCP operation modifies client IP address request packets to the extent needed to forward the packets to a DHCP server. Option 82 enhances this operation by enabling the routing switch to append an Option 82 field to such client requests. This field includes two suboptions for identifying both the routing switch (by MAC address or IP address) and the routing switch port the client is using to access the network. A DHCP server with Option 82 capability can read the appended field and use this data as criteria for selecting the IP addressing it will return to the client through the usual DHCP server response packet.

 

DHCP Option 82 provides several advantages over DHCP without Option 82:

 

■ An Option 82 DHCP server can use a relay agent’s identity and client

 

source port information to administer IP addressing policies based

 

on client and relay agent location within the network, regardless of

 

whether the relay agent is the client’s primary relay agent or a

 

secondary agent.

 

■ A routing switch operating as a primary Option 82 relay agent for

 

DHCP clients requesting an IP address can enhance network access

 

protection by blocking attempts to use an invalid Option 82 field to

 

imitate an authorized client, or by blocking attempts to use response

 

packets with missing or invalid Option 82 suboptions to imitate valid

 

response packets from an authorized DHCP server.

 

■ An Option 82 relay agent can also eliminate unnecessary broadcast

 

traffic by forwarding an Option 82 DHCP server response only to the

 

port on which the requesting client is connected, instead of broad­

 

casting the DHCP response to all ports on the VLAN.

 

 

N o t e

The routing switch’s DHCP Relay Information (Option 82) feature can be used

 

in networks where the DHCP server(s) are compliant with RFC 3046 Option

 

82 operation. DHCP Servers that are not compliant with Option 82 operation

 

ignore Option 82 fields. For information on configuring an Option 82 DHCP

 

server, refer to the documentation provided with the server application.

 

Some client applications can append an Option 82 field to their DHCP

 

requests. Refer to the documentation provided for your client application.

 

It is not necessary for all relay agents on the path between a DHCP client and

 

 

the server to support Option 82, and a relay agent without Option 82 should

 

forward DHCP packets regardless of whether they include Option 82 fields.

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