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IP Routing Features
Configuring DHCP Relay
DHCP Option 82
Option 82 is called the Relay Agent Information option and is inserted by the
DHCP relay agent when forwarding client-originated DHCP packets to a
DHCP server. Servers recognizing the Relay Agent Information option may
use the information to implement IP address or other parameter assignment
policies. The DHCP Server echoes the option back verbatim to the relay agent
in server-to-client replies, and the relay agent strips the option before
forwarding the reply to the client.
The “Relay Agent Information” option is organized as a single DHCP option
that contains one or more “sub-options” that convey information known by
the relay agent. The initial sub-options are defined for a relay agent that is co-
located in a public circuit access unit. These include a “circuit ID” for the
incoming circuit, and a “remote ID” which provides a trusted identifier for the
remote high-speed modem.
The routing switch can operate as a DHCP relay agent to enable communica-
tion between a client and a DHCP server on a different subnet. Without 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 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. This operation 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 broadcasting the
DHCP response to all ports on the VLAN.
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