Hop count in DHCP requests
When a DHCP client broadcasts requests, the DHCP relay agent in the routing switch receives the packets and
forwards them to the DHCP server (on a different subnet, if necessary.) During this 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.
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Aruba 3810 / 5400R Multicast and Routing Guide for ArubaOS-
Switch 16.08
