TP-Link T2600G-28TS User Manual page 217

To allow all clients in different VLAN request IP address from one server successfully, the DHCP
Relay function can transmit the DHCP packet between clients and server in different VLANs, and
all clients in different VLANs can share one DHCP Server.
When receiving DHCP-DISCOVER and DHCP-REQUEST packets, the switch will fill the

giaddr field with the interface IP of the receiving port, optionally insert the option 82
information, and then forward the packet to the server.
When receiving DHCP-OFFER and DHCP-ACP packets from the server, the switch will

delete the option 82 information and forward the packet to the interface which receives the
request.
The process will be 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 10-22 DHCP Relay Process
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