Dhcp snooping option 82 configuration

DHCP Snooping Option 82 Configuration Examples
This document describes the typical application environment and configuration
examples for DHCP snooping Option 82.

Acronyms:
Acronym Full spelling
DHCP Dynamic Host Configuration Protocol
DNS Domain Name System
giaddr Gateway IP address
WINS Windows Internet Naming Service

1 Feature Overview
Option 82 is the relay agent option which records the location information of the
DHCP client. When a DHCP snooping device receives a client’s request, it adds Option
82 to the request message and sends it to the server. Then, the DHCP server can
assign a proper IP address and other parameters for the client. The administrator can
also use Option 82 to implement security control and accounting.

2 Application Scenarios

Figure 1 Option 82 application
Typically, a DHCP server assigns an IP address based on the giaddr filed of the client’s
request or the IP address of the interface that received the client’s request. In Figure
1, the DHCP server assign IP addresses to Host A and Host B from the network
segment where the clients belong.
http://blog.router-switch.com/

Traditionally, the DHCP server cannot assign to Host A an IP address that is in a
different network segment from the IP address assigned to Host B. However, this can
be achieved through Option 82, with which, the DHCP server can assign IP addresses
based on the DHCP snooping interface connected to the clients and the giaddr filed
in DHCP requests.

A client’s ID can be recognized by Option 82. Therefore, the DHCP server can assign a
unique IP address to each client, to further implement QoS, security and accounting
management.

3 Configuration Guidelines
 The DHCP snooping Option 82 function can take effect only after you enable
DHCP snooping.
 DHCP snooping does not support link aggregation. If a Layer 2 Ethernet interface
is added into an aggregation group, DHCP snooping configuration on it will not
take effect. When the interface is removed from the group, DHCP snooping can
take effect.
 The DHCP snooping enabled device does not work if it resides between a DHCP
relay agent and DHCP server, and it can work when it resides between a DHCP
client and relay agent or between a DHCP client and server.
 You are recommended to enable the DHCP snooping Option 82 function on the
DHCP snooping device closest to the DHCP client for locating the client
accurately.
 The DHCP snooping enabled device cannot act as a DHCP server or DHCP relay
agent.
 You are not recommended to enable the DHCP client, BOOTP client, and DHCP
snooping on the same device. Otherwise, DHCP snooping entries may fail to be
generated, or the BOOTP client/DHCP client may fail to obtain an IP address.

4 Configuration Examples
4.1 Network Requirements
The work area of an enterprise is divided into three groups, group 1, group 2, and
group 3, which are located in three rooms. A DHCP server is deployed to assign IP
addresses of different segments to the three groups.
It is required that:
The DHCP server assigns IP addresses on the network segment 192.168.10.0/24
to devices in the work area. The lease time is 12 hours, and the DNS and WINS
server addresses are 192.168.100.2 and 192.168.100.3 respectively.
Group 1, group 2 and group 3 are connected to the DHCP snooping device
through Ethernet1/1, Ethernet1/2 and Ethernet1/3 respectively to communicate
with the DHCP server.
The DHCP server assigns IP addresses ranging from 192.168.10.2 to
192.168.10.25 to clients in group 1, assigns IP addresses ranging from
192.168.10.100 to 192.168.10.150 to clients in group 2, and assigns IP addresses

ranging from 192.168.10.151 to 192.168.10.200 to clients in group 3.

Figure 2 Network diagram for DHCP snooping
4.2 Configuration Considerations
 Enable Option 82 support on the DHCP snooping device.
 Configure the Option 82 sub-option, so that the clients in different groups can
send packets carrying different Option 82 information. To do so, you can
manually specify the circuit ID sub-option.
 Configure IP address assignment policy on the DHCP server, so that the DHCP
server can assign proper IP addresses to the DHCP clients according to Option
82.

4.3 Software Version Used
This example is configured and verified on Comware V500R002B42D001.

4.4 Configuration Procedures
Note:The following configurations are made on devices that are using default
settings and verified in a lab environment. When using the following configurations
on your devices in a live network, make sure they do not conflict with your current
configurations to prevent potential negative impact on your network.

4.4.1 Configuration on the DHCP Snooping Device
I. Configuration steps
# Enable DHCP snooping.
<Switch> system-view
[Switch] dhcp-snooping
# Configure Ethernet 1/4 as a DHCP snooping trusted port.
[Switch] interface ethernet 1/4

[Switch-Ethernet1/4] dhcp-snooping trust
[Switch-Ethernet1/4] quit
# Enable Ethernet 1/1 to support Option 82.
[Switch-Ethernet1/1] dhcp-snooping information enable
# Pad the Option 82 circuit ID sub-option with group 1.
[Switch-Ethernet1/1] dhcp-snooping information circuit-id string group1

II. Configuration file
<Switch> display current-configuration
#
interface Ethernet1/1
port link-mode bridge
dhcp-snooping information enable
dhcp-snooping information circuit-id string group1
#
#
#
dhcp-snooping trust
#

4.4.2 Configuration on the DHCP Server
I. Configuration steps
You can use the following two methods to configure Option 82:
1. User-defined method: Manually specify the content of Option 82.
2. Non-user-defined method: Pad Option 82 in the default normal or verbose
format.

For the second method, the circuit ID sub-option format is as shown in Figure 3 .

Figure 3 Circuit ID sub-option format
For example, for clients connected to Ethernet 1/1, the circuit ID sub-option is
padded with group 1. The circuit ID sub-option in DHCP packets should contain the
following information: 0x010667726F757031, in which 0106 refers to the number
and length of the circuit ID sub-option, and 67726F757031 refers to the hexadecimal
value of the character string group 1.

In this example, IP addresses are assigned according to the group number; therefore,
the DHCP server only needs to assign IP addresses based on the group number
padded in the circuit ID sub-option.

Note:The DHCP server is configured on a Cisco Catalyst 3745 switch with software
version IOS 12.3(11)T2. To configure a device of another type or version as the DHCP
server, refer to the related user manual.

# Configure the server interface IP address as 192.168.10.1/24.
Server> enable
Server# configure terminal
Server(config)# interface fastethernet 0/0
Server(config-if)# ip address 192.168.10.1 255.255.255.0
Server(config-if)# exit
# Enable DHCP server, and configure the DHCP server to assign IP addresses based on
Option 82.
Server(config)# service dhcp
Server(config)# ipdhcp use class
# Create a DHCP class for clients in group 1, and specify the corresponding circuit ID
sub-option for matching. For the content not to be matched, enter the wildcard ”*”.
Server(config)# ipdhcp class group1
Server(dhcp-class)# relay agent information
Server(dhcp-class-relayinfo)# relay-information hex 010667726F757031*
Server(dhcp-class-relayinfo)# exit

# Create a DHCP class for clients in group 2, and specify the corresponding circuit ID
sub-option for matching.
# Create a DHCP class for clients in group 3 and specify the corresponding circuit ID
sub-option for matching.
# Create a DHCP address pool named office, and specify the lease time, gateway
address, DNS server address, and WINS server address for clients.
Server(config)# ipdhcp pool office
Server(dhcp-config)# network 192.168.10.0
Server(dhcp-config)# lease 0 12
Server(dhcp-config)# default-router 192.168.10.1
Server(dhcp-config)# dns-server 192.168.100.2
Server(dhcp-config)# netbios-name-server 192.168.100.3
# Specify address ranges for the three DHCP classes respectively.
Server(dhcp-config)# class group1
Server(dhcp-pool-class)# address range 192.168.10.2 192.168.10.25
Server(dhcp-pool-class)# class group2
Server(dhcp-pool-class)# class group3

4.4.3 Verification
After completing the above configurations, the DHCP server can automatically assign
IP addresses of the specified range, gateway address, DNS server address, and WINS
server address for clients of each group in the work area.

More Related DHCP Snooping Tips:
How to Configure DHCP Snooping?


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