How The Relay Agent Processes Dhcp Packets From The Client - Alcatel-Lucent OmniSwitch AOS Release 7 Manual

Configuring DHCP Relay
For more information about using the ip udp relay service vlan command, see the OmniSwitch CLI
Reference Guide.

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
When the relay agent receives a DHCP packet from the DHCP server, the agent:
1
Extracts the VLAN ID from the Circuit ID suboption field in the packet and compares 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
Drops the DHCP packet if the IP router interface MAC address and the Remote ID MAC address are
not the same.
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 tries to deliever the packet back to the port where the device is located.
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.
OmniSwitch AOS Release 7 Network Configuration Guide
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
policy 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.
March 2011
Configuring UDP Port Relay
command.
page 17-15