Multicast and Routing Guide K/KA/KB.15.15

process, the DHCP relay agent increments the hop count before forwarding DHCP packets to the
server. The DHCP server, in turn, includes the hop count from the received DHCP request in the
response sent back to a DHCP client.
As a result, the DHCP client receives a non-zero hop count in the DHCP response packet. Because
some legacy DHCP/BootP clients discard DHCP responses that contain a hop count greater than
one, they may fail to boot up properly. Although this behavior is in compliance with RFC 1542,
it prevents a legacy DHCP/BootP client from being automatically configured with a network IP
address.
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 suboptions that convey information known by the relay agent. The initial suboptions 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 that provides a trusted identifier for the remote high-speed
modem.
The routing switch can operate as a DHCP relay agent to enable communication 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.
NOTE: The routing switch's DHCP relay information (Option 82) feature can be used in networks
where the DHCP servers are compliant with RFC 3046 Option 82 operation. DHCP servers that
are not compliant with Option 82 operation ignore Option 82 fields.
Some client applications can append an Option 82 field to their DHCP requests.
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. However, Option 82 relay agents should be positioned at
the DHCP policy boundaries in a network to provide maximum support and security for the IP
addressing policies configured in the server.
238 Dynamic Host Configuration Protocol