How The Relay Agent Processes Dhcp Packets From The Client; How The Relay Agent Processes Dhcp Packets From The Server - Alcatel-Lucent OmniSwitch 6800 Series Network Configuration Manual

Configuring DHCP Security Features

How the Relay Agent Processes DHCP Packets from the Client

The following table describes how the relay agent processes DHCP packets received from clients when the
Option-82 feature is enabled for the switch:
If the DHCP packet from the client ...
Contains a zero gateway IP address (0.0.0.0) and
no Option-82 data.
Contains a zero gateway IP address (0.0.0.0) and
Option-82 data.
Contains a non-zero gateway IP address and no
Option-82 data.
Contains a non-zero gateway IP address and
Option-82 data.

How the Relay Agent Processes DHCP Packets from the Server

Note that if a DHCP server does not support Option-82, the server strips the option from the packet. If the
server does support this option, the server will retain the Option-82 data received and send it back in a
reply packet.
When the relay agent receives a DHCP packet from the DHCP server and the Option-82 feature is
enabled, the agent will:
1
Extract the VLAN ID from the Circuit ID suboption field in the packet and compare the MAC address
of the IP router interface for that VLAN to the MAC address contained in the Remote ID suboption field
in the same packet.
2
If the IP router interface MAC address and the Remote ID MAC address are not the same, then the
agent will drop the packet.
3
If the two MAC addresses match, then a check is made to see if the slot/port value in the Circuit ID
suboption field in the packet matches a port that is associated with the VLAN also identified in the Circuit
ID suboption field.
4
If the slot/port information does not identify an actual port associated with the Circuit ID VLAN, then
the agent will drop the packet.
5
If the slot/port information does identify an actual port associated with the Circuit ID VLAN, then the
agent strips the Option-82 data from the packet and unicasts the packet to the port identified in the Circuit
ID suboption.
page 22-16
The relay agent ...
Inserts Option-82 with unique information to
identify the client source.
Drops the packet, keeps the Option-82 data and
forwards the packet, or replaces the Option-82
data with its own Option-82 data and forwards the
packet.
The action performed by the relay agent in this
case is determined by the agent information pol-
icy that is configured through the
ip helper agent-information policy
By default, this type of DHCP packet is dropped
by the agent.
Drops the packet without any further processing.
Drops the packet if the gateway IP address
matches a local subnet, otherwise the packet is
forwarded without inserting Option-82 data.
OmniSwitch 6800/6850/9000 Network Configuration Guide
Configuring DHCP Relay
command.
March 2008