User Guide 398
Configuring DHCP Service DHCP
*Value
Indicates the value of the sub-option. The switch has preset a default circuit ID and
remoter ID. You can also customize them with Circuit ID Customization and Remote ID
Customization enabled.
■ Default circuit ID: A 4-byte value which consists of 2-byte VLAN ID and 2-byte Port
ID. The VLAN ID indicates which VLAN the DHCP client belongs to, and the Port ID
indicates which port the DHCP client is connected to.
For example, if the DHCP client is connected to port 1/0/1 in VLAN 2, this field is
00:02:00:01 in hexadecimal.
■ Default remote ID: A 6-byte value which indicates the MAC address of the DHCP relay
agent.
■ Customized circuit/remote ID: You can configure a string using up to 64 characters. The
switch encodes the string using ASCII. When configuring your DHCP server to identify
the string, use the correct notation that is used by your DHCP server to represent ASCII
strings, or convert it into hexadecimal format if necessary.
Tips:
As shown in Table 1-1 and Table 1-2, by default, the circuit ID records the ports of the
DHCP relay agent that are connected to the clients and the VLANs that the clients belong
to, and the remote ID records the MAC address of the DHCP relay agent. That is, the two
sub-options together record the location of the clients. To record the accruate location of
clients, configure Option 82 on the switch which is closest to the clients.
■ DHCP VLAN Relay
DHCP VLAN Relay allows clients in different VLANs to obtain IP addresses from the DHCP
server using the IP address of a single agent interface.
In DHCP VLAN Relay, you can simply specify the default management VLAN interface as
the default agent interface for all VLANs. The switch fills this default agent interface’s IP
address in the Relay Agent IP Address field of the DHCP packets from all VLANs.
As the following figure shows, no IP addresses are assigned to VLAN 10 and VLAN 20,
but a default relay agent interface is configured with the IP address 192.168.2.1/24. The
switch fills in the Relay Agent IP Address field of the DHCP packets with the IP address
of the default agent interface (192.168.2.1/24) when applying for IP addresses for clients
in both VLAN 10 and VLAN 20. As a result, the DHCP server will assign IP addresses on
192.168.2.0/24 (the same subnet with the IP address of the default agent interface) to
clients in both VLAN 10 and VLAN 20.
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