What Is Option 43 in DHCP?
DHCP Overview
The Dynamic Host Configuration Protocol (DHCP) dynamically configures and uniformly manages IP addresses of hosts. DHCP is defined in RFC 2131 and uses the client/server communication model. A DHCP client requests configuration information from a DHCP server, and the DHCP server returns the configuration information allocated to the DHCP client.
A DHCP server communicates with DHCP clients by exchanging DHCP messages. Figure 19-33 shows the DHCP message format. Digits in the parentheses indicate the field length, in octets.
DHCP Option Field
A variable options field in a DHCP message is used to store control information and parameters allocated to the DHCP client. As shown in Figure 19-34, the options field consists of three parts: Type, Length, and Value. Table 19-32 describes the three parts.
Field |
Length |
Description |
|---|---|---|
Type |
1 octet |
Information type |
Length |
1 octet |
Length of the information content |
Value |
Depending on the Length option |
Information content |
The value of the DHCP options field ranges from 1 to 255. DHCP options include predefined and user-defined options.
Option 43
Option 43 is a user-defined option, indicating vendor-specific information. A device that supports Option 43 is used to connect to different terminals such as IP phones and APs.
Figure 19-35 shows the format of Option 43.
DHCP servers and clients use Option 43 to exchange vendor-specific information. When a DHCP server receives a DHCP request message with parameter 43 encapsulated in Option 55, it encapsulates Option 43 in a reply message and sends the message to the DHCP client.
Option 43 supports suboptions, as shown in Figure 19-35.
- Sub-option type: type of the suboption
- Sub-option length: length of the suboption
- Sub-option value: value of the suboption