TP-Link T2500-28TC User Manual page 149

When receiving DHCP-OFFER and DHCP-ACK packets from the server, the switch will

delete the option 82 information and forward the packets to the port which receives the
request.
The process is shown as follows.
DHCP Relay Configuration

1) Configure the Option 82 parameters to record the information of the clients. You are
suggested to configure the option82 on the nearest Relay of the client.
2) Specify the DHCP Server which assigns IP addresses actually.
Option 82

On this switch, Option 82 is used to record the location of the DHCP Client, the Ethernet port and
the VLAN, etc. Upon receiving the DHCP-REQUEST packet, the switch adds the Option 82 field to
the packet and then transmits the packet to DHCP Server. The Server can be acquainted with the
location of the DHCP Client via Option 82, so as to locate the DHCP Client, and assign the
distribution policy of IP addresses and the other parameters for fulfilling the security control and
account management of the client.
Option 82 can contain 255 sub-options at most. If Option 82 is defined, at least one sub-option
should be defined. This Switch supports two sub-options, Circuit ID and Remote ID. Since there is
no universal standard about the content of Option 82, different manufacturers define the
sub-options of Option 82 to their need. For this Switch, the sub-options are defined as follows:
The Circuit ID is defined to be the number and VLAN of the port which receives the DHCP
Request packets. The Remote ID is defined to be the MAC address of DHCP Relay device which
receives the DHCP Request packets from DHCP Clients. Furthermore these two parameters also
can be manually configured.
The format of Option 82 defined on the switch by default is given in the following figure. The
numbers in parentheses indicate the size of each field in octets. By default, sub-option1 is Circuit
ID option recording the VLAN and Ethernet port information, while sub-option2 is Remote ID
option recording the MAC address information of the client. You can define the sub-options
manually.
Figure 9-6 DHCP Relay Process
Figure9-7 Option 82
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