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Configuration Guide - IP Service

CloudEngine 8800, 7800, 6800, and 5800 V200R003C00

This document describes the configurations of IP Service, including IP address, ARP, DHCP, DNS, IP performance optimization, IPv6, DHCPv6, and IPv6 DNS.
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DHCP Options

DHCP Options

Options Field in a DHCP Message

The Options field in a DHCP message carries control information and parameters that are not defined in common protocols. When a DHCP client requests an IP address from a DHCP server configured with the Options field, the server replies a message containing the Options field. Figure 3-3 shows the format of the Options field.

Figure 3-3 Format of the Options field

The Options field consists of Type, Length, and Value. The following table provides the details.

Table 3-3 Description of the Options field

Field

Length

Description

Type

1 byte

Indicates the type of the message content.

Length

1 byte

Indicates the length of the message content.

Value

Variable

Indicates the message content. The length varies depending on the Length field.

The value of the Options field ranges from 1 to 255. Table 3-4 lists common DHCP options.

Table 3-4 Description of the Options field in DHCP messages

Options No.

Function

1

Specifies the subnet mask.

3

Specifies the gateway address.

6

Specifies the DNS server IP address.

12

Specifies the hostname of a DHCP client.

15

Specifies the domain name suffix.

33

Specifies a group of classful static routes. When a DHCP client receives DHCP packets with this option, it adds the classful static routes contained in the option to its routing table. In classful routes, masks of destination addresses are natural masks and masks cannot be used to divide subnets. If Option 121 exists, Option 33 is ignored.

44

Specifies the NetBIOS server name.

46

Specifies the NetBIOS object type.

50

Specifies the requested IP address.

51

Specifies the IP address lease time.

52

Specifies the additional option.

53

Specifies the DHCP message type.

54

Specifies the server identifier.

55

Specifies the requested parameter list. It is used by a DHCP client to request specified configuration parameters.

58

Specifies the lease renewal time (T1), which is 50% of the lease time.

59

Specifies the lease renewal time (T2), which is 87.5% of the lease time.

60

Specifies the vendor classification information, which identifies the DHCP client type and configuration.

61

Specifies the client identifier.

66

Specifies the TFTP server name allocated to DHCP clients.

67

Specifies the startup file name allocated to DHCP clients.

77

Specifies the user type.

121

Specifies a group of classless routes. After a DHCP client receives DHCP messages with this option, it adds the classless static routes contained in the option to its routing table. Classless routes are routes of which masks of destination addresses can be any values and masks can be used to divide subnets.

The objects of this field vary depending on the functions of the Options field. For example, Option 77 is used on a DHCP client to identify user types of the DHCP client. The DHCP server selects an address pool to allocate an IP address and configuration parameters to the DHCP client based on the User Class in the Option field. Option 77 is manually configured only on a DHCP client but not on the server.

For more information about common DHCP options, see related RFC documents.

Customized DHCP Options

Some options, such as Option 82, are not defined in RFC and can be customized.

The Option 82 field is called the DHCP relay agent information field. It records the location of a DHCP client. A DHCP relay agent or a DHCP snooping-enabled device appends the Option 82 field to a DHCP Request message sent from a DHCP client, and then forwards the DHCP Request message to a DHCP server.

An administrator can use the Option 82 field to locate a DHCP client and implement control over security and accounting of the DHCP client. According to information in the Option 82 field, a DHCP server can make a policy for allocating IP addresses and other parameters and provide flexible address allocation schemes.

The Option 82 field contains a maximum of 255 suboptions. If the Option 82 field is defined, at least one suboption must be defined.

The content of the Option 82 field is not uniformly defined, and vendors fill in the Option 82 field as needed. The device supports the following Option 82 field formats:
  • Type1: This is the Telecom format of Option 82.

  • Type2: This is the NMS format of Option 82.

  • Cn-telecom: This is the Option 82 format defined by China Telecom.

  • Self-define: This is the user-defined format of DHCP Option 82.

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Updated: 2019-05-08

Document ID: EDOC1100004354

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