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User Guide 548
Configuring DHCP Service DHCP
*Format
Indicates the packet format of the sub-option field. Two options are available:
â– Normal: Indicates the field consists of three parts: Type, Length, and Value (TLV).
â– Private: Indicates the field consists of the value only.
*Type
A one-byte field indicating whether the Value field is customized or not. 00 in hexadecimal
means the Value field is not customized (uses the default circuit/remote ID) while 01 in
hexadecimal means it is customized.
*Length
A one-byte field indicating the length of the Value field. The length of the default circuit ID
is 4 bytes and that of default remote ID is 6 bytes. For the customized circuit ID and remote
ID, the length is variable, ranging from 1 to 64 bytes.
*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.
â– 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.
â– Default remote ID: A 6-byte value which indicates the MAC address of the DHCP relay
agent.
â– 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.
â– DHCP Interface Relay
DHCP Interface Relay allows clients to obtain IP addresses from a DHCP server in a
different LAN. In DHCP Interface Relay, you can specify a DHCP server for the Layer 3
interface that the clients are connected to. When receiving DHCP packets from clients, the
switch fills the corresponding interface’s IP address in the Relay Agent IP Address field of
the DHCP packets, and forwards the packets to the DHCP server. Then the DHCP server
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