Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) User Manual
Page 1695
Dynamic Host Configuration Protocol (DHCP) Introduction
Software Reference for x310 Series Switches
C613-50046-01 REV A
AlliedWare Plus
TM
Operating System - Version 5.4.4C
63.9
If Option 82 insertion is enabled, then the DHCP packet flow is as follows:
■
The DHCP client generates a DHCP request and broadcasts it on the network.
■
The DHCP relay agent intercepts the broadcast DHCP request packet and inserts the
Option 82 field in the packet.
■
The DHCP relay agent unicasts the DHCP request that includes the Option 82 field to
the DHCP server.
■
The DHCP server receives the packet.
■
If the DHCP server supports Option 82, then it echoes the Option 82 field in the DHCP
reply. If the server does not support Option 82, it ignores the option and does not
echo it in the reply.
■
The DHCP server unicasts the reply to the relay agent.
■
The relay agent removes the Option 82 field and forwards the packet to the switch
port connected to the DHCP client that sent the DHCP request.
For information about DHCP Relay Agent Information Option (Option 82), see RFC 3046.
To enable the relay agent to insert its details into the Option 82 field in requests received
from clients on a particular interface, use the command:
The Option 82 field contains sub-options. You can specify a value for the Remote ID
sub-option, which contains information that identifies the host. To specify a value for the
Remote ID, use the command:
If a Remote ID value is not specified, the Remote ID sub-option is set to the switch’s MAC
address. You can also configure the Remote ID value as an alphanumeric string.
Note that the Option 82 agent information added by DHCP Relay differs from the
information inserted by DHCP snooping (see
“DHCP Relay Agent Option 82” on
Dealing with client-originated packets that already contain
Option 82 information
It is possible that the requests arriving from the clients to the relay agent could already
contain Option 82 data. There are two main circumstances in which this can occur:
1.
A client is maliciously inserting bogus information into the packet in an attempt to
subvert the process of identifying the client’s location. In this case, you would want to
drop the packets that contain the bogus information (or remove bogus information).
2.
A Layer 2 DHCP snooping switch, that sits between the clients and the DCHP relay, is
validly inserting the Option 82 information into the packets. The DHCP snooping
switch is not acting as a relay agent, but is inserting the Option 82 information. In this
case, you would want to forward the valid information to the DHCP server.
awplus(config)#
interface
awplus(config-if)#
awplus(config)#
interface
awplus(config-if)#