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Configuration Guide - IP Service

S1720, S2700, S5700, and S6720 V200R012(C00 and C20)

This document describes the configurations of IP Service, including IP address, ARP, DHCP, DHCP policy VLAN, DNS, mDNS gateway, mDNS relay, UDP Helper, IP performance optimization, IPv6, DHCPv6, IPv6 DNS, IPv6 over IPv4 tunnel, and IPv4 over IPv6 tunnel.
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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 DHCP server replies with a message containing the Options field. Figure 3-2 shows the format of the Options field.

Figure 3-2  Format of the Options field

The Options field consists of Type, Length, and Value, which are described in the following table.

Table 3-3  Description of the Options field





1 byte

Indicates the information type.


1 byte

Indicates the length of the subsequent content in the Options field.



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 well-known DHCP options.

Table 3-4  Well-known DHCP options in DHCP messages

Option No.



Specifies a subnet mask.


Specifies a gateway address.


Specifies the time server.


Specifies the IP address of a DNS server.


Specifies the log server.


Specifies the device name of a DHCP client.


Specifies a domain name.


Specifies the root path.


Specifies the broadcast address.


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


Specifies NTP servers.


Vendor specific information.


Specifies a NetBIOS name.


Specifies a NetBIOS node type.


Specifies a requested IP address.


Specifies an IP address lease.


Specifies an additional option.


Specifies a DHCP message type.


Specifies a server identifier.


Specifies the request option list. A DHCP client uses this option to request specified configuration parameters. The content of this option is the value of the option requested by the DHCP client.


Specifies a message option, which is used to describe the reason why an IP address fails to be allocated. This option is encapsulated into the Option field of the following DHCP packets.
  • NAK message sent by a DHCP server.
  • Decline or Release message sent by a DHCP client.
  • Decline message sent by a DHCP relay agent when the DHCP relay agent detects an IP address conflict or Release message sent by a DHCP client to release its IP address.
  • Release message sent by a DHCP snooping device to release its IP address.


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


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


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


Specifies a client identifier.


Specifies a TFTP server name allocated to DHCP clients.


Specifies a startup file name allocated to DHCP clients.


Specifies a user type.


Specifies the IP address option of the SIP server.

Currently, only IP addresses can be parsed and domain names cannot be parsed.


Specifies a group of classless routes. This option contains a group of classless static 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. In classless routes, masks of destination addresses can be any value and can be used to divide subnets.


A device functioning as a DHCP client can receive static routes delivered from a DHCP server through Option 121.


Specifies the IP address of call server.


Specifies a reserved option. You can customize information carried in this option.

The preceding options function on different objects. 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 Options field. Option 77 is manually configured only on the DHCP client but not on the server.

For more information about well-known DHCP options, see RFC 2132.

User-defined DHCP Options

Some options, such as Option 82, are not defined in RFC 2132 and are user-defined.

Option 82 is the DHCP relay agent information option that records the location of a DHCP client. A DHCP relay agent or a device with DHCP snooping enabled appends the Option 82 field to a DHCP Discover message sent from a DHCP client and then forwards the DHCP Discover message to a DHCP server.

An administrator can use the Option 82 field to locate a DHCP client and control the security and accounting of the DHCP client. A DHCP server that supports the Option 82 field can determine policies to flexibly allocate IP addresses and other parameters according to information in this field.

Option 82 contains a maximum of 255 sub-options. If the field is defined, at least one sub-option must be defined. Sub-options that are currently supported by the device include:
  • sub-option1(Agent Circuit ID Sub-option)
  • sub-option2(Agent Remote ID Sub-option)
  • sub-option6(Subscriber-ID Suboption)
  • sub-option9(Vendor-Specific Information Suboption)

The content of the Option 82 field is not defined uniformly. Therefore, vendors fill in the Option 82 field as required.

Updated: 2018-12-24

Document ID: EDOC1100038342

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