H3C Low-End Ethernet Switches Configuration Examples(V1.04)

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01-DHCP Configuration Examples
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DHCP Configuration Examples

Keywords: DHCP, Option 82

Abstract: This document describes DHCP configuration and application on Ethernet switches in specific networking environments. Based on the different roles played by the devices in the network, the functions and applications of DHCP server, DHCP relay agent, DHCP snooping, and DHCP Option 82 are covered.

Acronym: DHCP (Dynamic Host Configuration Protocol).

 


Chapter 1  DHCP Functions Overview

1.1  Supported DHCP Functions

1.1.1  DHCP Functions Supported by the H3C Low-End Ethernet Switches

Table 1-1 DHCP functions supported by the H3C low-end ethernet switches

Function

Model

DHCP server

DHCP relay agent

DHCP snooping

S3600-EI

l

l

l

S3600-SI

l

l

S5600

l

l

l

S3610

l

l

l

S5510

l

l

l

S5500-SI

l

l

S5100

l

S3100

l

 

Depending on the models, the H3C low-end switches can support part or all of the following DHCP functions:

DHCP server:

l           DHCP server using global address pool/interface address pool

l           IP address lease configuration

l           Allocation of gateway addresses, DNS server addresses, WINS server addresses to DHCP clients

l           Static bindings for special addresses

l           DHCP server security functions: detection of unauthorized DHCP servers and detection of duplicate IP addresses

DHCP relay agent:

l           DHCP relay agent

l           DHCP relay agent security functions: address checking, DHCP server handshake, and periodic update of client address entries

l           DHCP Option 82

DHCP snooping:

l           DHCP snooping

l           DHCP snooping security functions: DHCP snooping entry update and ARP source checking

l           DHCP Option 82

 

&  Note:

 Refer to respective user manuals for detailed descriptions of the DHCP functions supported by different models.

 

1.2  Configuration Guide

 

&  Note:

l      The configuration varies with product models. The following configuration takes the S3600 series as an example. Refer to respective operation manuals for the configurations on other models.

l      Only basic configuration steps are listed below. Refer to respective operation and command manuals for the operating principles and applications of the functions.

 

1.2.1  Configuring the DHCP Server

The DHCP server can be configured to assign IP addresses from a global or interface address pool. These two configuration methods are applicable to the following environments:

l           If the DHCP server and DHCP clients are on the same network segment, both methods can be applied.

l           If the DHCP server and DHCP clients are on different network segments, the DHCP server can only be configured to assign IP addresses from a global address pool.

1)         Use the following commands to configure the DHCP server to assign IP addresses from a global address pool.

Table 1-2 Configure IP address allocation from a global address pool

Operation

Command

Description

Enter system view

system-view

Enable the DHCP service

dhcp enable

Optional

By default, the DHCP service is enabled.

Create a DHCP address pool and enter DHCP address pool view

dhcp server ip-pool pool-name

Required

By default, no global DHCP address pool is created.

Configure an IP address range for dynamic allocation

network ip-address [ mask-length | mask mask ]

Required

By default, no IP address range is configured for dynamic allocation.

Configure the lease period of dynamically allocated IP addresses

expired { day day [ hour hour [ minute minute ] ] | unlimited }

Optional

IP address lease period defaults to one day.

Configure a domain name for DHCP clients

domain-name domain-name

Required

By default, no domain name is configured for DHCP clients.

Configure DNS server addresses for DHCP clients

dns-list ip-address&<1-8>

Required

By default, no DNS server addresses are configured.

Configure WINS server addresses for DHCP clients

nbns-list ip-address&<1-8>

Required

By default, no WINS server addresses are configured.

Specify a NetBIOS node type for DHCP clients

netbios-type { b-node | h-node | m-node | p-node }

Optional

By default, the DHCP clients are h-nodes if the command is not specified.

Configure gateway addresses for DHCP clients

gateway-list ip-address&<1-8>

Required

By default, no gateway address is configured.

Configure a self-defined DHCP option

option code { ascii ascii-string | hex hex-string&<1-10> | ip-address ip-address&<1-8> }

Required

By default, no self-defined option is configured.

Configure a static binding

Return to system view

quit

Optional

By default, no MAC address or client ID is bound to an IP address statically.

Note:

l      To configure a static binding, you need to specify the IP address and the MAC address or client ID.

l      A static address pool can be configured with only one IP address-to-MAC or IP address-to-client ID binding.

Create an address pool for the static address binding

dhcp server ip-pool pool-name

Specify the IP address of the static binding

static-bind ip-address ip-address [ mask-length | mask mask ]

Specify the MAC address or the client ID of the static binding

Specify the MAC address of the static binding

static-bind mac-address mac-address

Specify the client ID of the static binding

static-bind client-identifier client-identifier

Return to system view

quit

Specify the IP addresses to be excluded from automatic allocation

dhcp server forbidden-ip low-ip-address [ high-ip-address ]

Optional

By default, all the IP addresses in a DHCP address pool are available for dynamic allocation.

Configure the global address pool mode

On the current interface

interface interface-type interface-number

Optional

By default, an interface operates in the global address pool mode.

dhcp select global

quit

On multiple interfaces in system view

dhcp select global { interface interface-type interface-number [ to interface-type interface-number ] | all }

Enable the detection of unauthorized DHCP servers

dhcp server detect

Required

By default, the detection of unauthorized DHCP servers is disabled.

Configure duplicate IP address detection

Set the maximum number of ping packets sent by the DHCP server for each IP address

dhcp server ping packets number

Optional

The default maximum number is 2.

Set a response timeout for each ping packet

dhcp server ping timeout milliseconds

Optional

The default timeout is 500 milliseconds.

Enable the DHCP server to support Option 82

dhcp server relay information enable

Optional

By default, the DHCP server supports Option 82.

 

2)         Use the following commands to configure IP address allocation through the interface address pool.

Table 1-3 Configure IP address allocation through the interface address pool

Operation

Command

Description

Enter system view

system-view

Enable the DHCP service

dhcp enable

Optional

By default, the DHCP service is enabled.

Configure multiple or all the VLAN interfaces to operate in interface address pool mode

dhcp select interface { interface interface-type interface-number [ to interface-type interface-number ] | all }

Optional

Configure a VLAN interface to operate in interface address pool mode

interface interface-type interface-number

Required

By default, a VLAN interface operates in global address pool mode.

dhcp select interface

Bind an IP address statically to a client MAC address or client ID

dhcp server static-bind ip-address ip-address { client-identifier client-identifier | mac-address mac-address }

Optional

By default, no static binding is configured

Configure the lease period of dynamically allocated IP addresses

On the current interface

dhcp server expired { day day [ hour hour [ minute minute ] ] | unlimited }

Optional

IP address lease period defaults to one day.

On multiple interfaces in system view

quit

dhcp server expired { day day [ hour hour [ minute minute ] ] | unlimited } { interface interface-type interface-number [ to interface-type interface-number ] | all }

Return to system view

quit

Specify the IP addresses to be excluded from automatic allocation

dhcp server forbidden-ip low-ip-address [ high-ip-address ]

Optional

By default, all the IP addresses in an interface address pool are available for dynamic allocation.

Configure a domain name for DHCP clients

On one interface

interface interface-type interface-number

Optional

By default, no domain name is configured for DHCP clients.

dhcp server domain-name domain-name

quit

On multiple interfaces

dhcp server domain-name domain-name { interface interface-type interface-number [ to interface-type interface-number ] | all }

Configure DNS server addresses for DHCP clients

On one interface

interface interface-type interface-number

Optional

By default, no DNS server address is configured.

dhcp server dns-list ip-address&<1-8>

quit

On multiple interfaces

dhcp server dns-list ip-address&<1-8> { interface interface-type interface-number [ to interface-type interface-number ] | all }

Configure WINS server addresses for DHCP clients

On one interface

interface interface-type interface-number

Optional

By default, no WINS server addresses are configured.

dhcp server nbns-list ip-address&<1-8>

quit

On multiple interfaces

dhcp server nbns-list ip-address&<1-8> { interface interface-type interface-number [ to interface-type interface-number ] | all }

Define a NetBIOS node type for DHCP clients

On one interface

interface interface-type interface-number

Optional

By default, no NetBIOS node type is specified and a DHCP client uses the h-node type.

dhcp server netbios-type { b-node | h-node | m-node | p-node }

quit

On multiple interfaces

dhcp server netbios-type { b-node | h-node | m-node | p-node } { interface interface-type interface-number [ to interface-type interface-number ] | all }

Configure a self-defined DHCP option

On one interface

interface interface-type interface-number

Optional

By default, no self-defined option is configured.

dhcp server option code { ascii ascii-string | hex hex-string&<1-10> | ip-address ip-address&<1-8> }

quit

On multiple interfaces

dhcp server option code { ascii ascii-string | hex hex-string&<1-10> | ip-address ip-address&<1-8> } { interface interface-type interface-number [ to interface-type interface-number ] | all }

Enable the detection of unauthorized DHCP servers

dhcp server detect

Optional

By default, the detection of unauthorized DHCP servers is disabled.

Configure duplicate IP address detection

Set the maximum number of ping packets sent by the DHCP server for each IP address

dhcp server ping packets number

Optional

The default maximum number is 2.

Set a response timeout for each ping packet

dhcp server ping timeout milliseconds

Optional

The default timeout is 500 milliseconds.

Enable the DHCP server to support Option 82

dhcp server relay information enable

Optional

By default, the DHCP server supports Option 82.

 

1.2.2  Configuring the DHCP Relay Agent

Use the following commands to configure the DHCP relay agent.

Table 1-4 Configure DHCP relay agent

Operation

Command

Description

Enter system view

system-view

Enable the DHCP service

dhcp enable

Optional

By default, the DHCP service is enabled.

Configure DHCP server IP addresses for a DHCP server group

dhcp-server groupNo ip ip-address&<1-8>

Required

By default, no DHCP server IP address is configured for a DHCP server group.

Configure a DHCP user address entry

dhcp-security static ip-address mac-address

Optional

By default, no DHCP user address entry is configured.

Enable DHCP relay agent handshake

dhcp relay hand enable

Optional

By default, DHCP relay agent handshake is enabled.

Configure the interval at which the DHCP relay agent updates dynamic client address entries

dhcp-security tracker { interval | auto }

Optional

By default, the update interval is calculated automatically according to the number of the DHCP client entries.

Enable the detection on unauthorized DHCP servers

dhcp-server detect

Required

By default, the detection of unauthorized DHCP servers is disabled.

Enable the DHCP relay agent to support Option 82

dhcp relay information enable

Required

By default, the DHCP relay agent does not support Option 82.

Configure a strategy for the DHCP relay agent to handle request packets containing Option 82

dhcp relay information strategy { drop | keep | replace }

Optional

By default, the strategy is replace.

Enter VLAN interface view

interface interface-type interface-number

Associate the interface to a DHCP server group

dhcp-server groupNo

Required

By default, a VLAN interface is not associated to any DHCP server group.

Enable the address checking function for the DHCP relay agent

address-check enable

Required

By default, the address checking function is disabled for the DHCP relay agent.

 

1.2.3  Configuring DHCP Snooping

Use the following commands to configure DHCP snooping:

Table 1-5 Configure DHCP snooping

Operation

Command

Description

Enter system view

system-view

Enable DHCP snooping

dhcp-snooping

Required

By default, DHCP snooping is disabled.

Enter Ethernet port view

interface interface-type interface-number

Specify the port connected to the DHCP server as a trusted port

dhcp-snooping trust

Optional

By default, all the ports of a switch are untrusted ports.

 


Chapter 2  Configuration Examples

2.1  DHCP Server Configuration Example

2.1.1  Network Requirements

An S3600 switch serves as the DHCP server in the corporate headquarters (HQ) to allocate IP addresses to the workstations in the HQ and Branch, and it also acts as the gateway to forward packets from the HQ. The network requirements are as follows:

l           Assign the HQ the IP addresses in the 10.214.10.0/24 network segment, with a lease period of two days, and exclude the IP addresses of the DNS server, WINS server, and mail server from allocation.

l           Assign IP addresses to the DNS server, WINS server, and the mail server in HQ through static bindings.

l           Assign the workstations in the Branch the IP addresses in the 10.210.10.0/24 network segment, with a lease period of three days, and assign the file server in the Branch an IP address through a static IP-to-MAC binding.

l           Assign the addresses of the gateway, DNS server, and the WINS server along with an IP address to each workstation in the HQ and Branch.

l           Enable the detection of unauthorized DHCP servers to prevent any unauthorized DHCP server from allocating invalid addresses.

2.1.2  Network Diagram

Figure 2-1 Network diagram for DHCP server configuration

2.1.3  Configuration Procedure

I. Software Version Used

The S3600 Ethernet switches running software version Release 1510 are used in this example.

II. Configuring DHCP server

l           Configure address allocation for the devices in the HQ.

# Configure the IP address of VLAN-interface10 on the DHCP server in the HQ.

<H3C> system-view

[H3C] interface Vlan-interface 10

[H3C-Vlan-interface10] ip address 10.214.10.1 24

# Configure the interface to operate in the interface address pool mode, assigning the IP addresses in the 10.214.10.0/24 network segment to the devices in the HQ.

[H3C-Vlan-interface10] dhcp select interface

# Configure the address lease period of the address pool, and configure the IP addresses of the DNS server and WINS server.

[H3C-Vlan-interface10] dhcp server expired day 2 

[H3C-Vlan-interface10] dhcp server dns-list 10.214.10.3

[H3C-Vlan-interface10] dhcp server nbst-list 10.214.10.4

No gateway needs to be configured for the clients because an interface operating in the interface address pool mode automatically serves as the gateway for DHCP clients and sends the requested information to the clients.

# Assign IP addresses to the DNS server, WINS server, and mail server through IP-to-MAC bindings.

[H3C-Vlan-interface10] dhcp server static-bind ip-address 10.214.10.3 mac-address 000d-85c7-4e20

[H3C-Vlan-interface10] dhcp server static-bind ip-address 10.214.10.4 mac-address 0013-4ca8-9b71

[H3C-Vlan-interface10] dhcp server static-bind ip-address 10.214.10.5 mac-address 002e08d20-54c6

# Exclude the static IP addresses of the DNS server, WINS server, and mail server from allocation.

[H3C-Vlan-interface10] quit

[H3C] dhcp server forbidden-ip 10.214.10.3 10.214.10.5

l           Configure address allocation for the devices in the Branch.

# Create a global address pool named “br” for the Branch, and specify the range and lease period of the IP addresses for allocation.

[H3C] dhcp server ip-pool br

[H3C-dhcp-pool-br] network 10.210.10.0 mask 255.255.255.0

[H3C-dhcp-pool-br] expired day 3 

# Create a static binding address pool named “br-static”, and assign the file server in the Branch an IP address through an IP-to-MAC binding.

[H3C-dhcp-pool-br] quit

[H3C] dhcp server ip-pool br-static

[H3C-dhcp-pool-br-static] static-bind ip-address 10.214.10.4 mask 255.255.255.0

[H3C-dhcp-pool-br-static] static-bind mac-address 000d-88f8-4e71

# Specify the gateway address, DNS server address, and the WINS server address for the workstations in the Branch.

[H3C-dhcp-pool-br-static] quit

[H3C] dhcp server ip-pool br

[H3C-dhcp-pool-br] gateway-list 10.210.10.1

[H3C-dhcp-pool-br] dns-list 10.214.10.3

[H3C-dhcp-pool-br] nbst-list 10.214.10.4

# Exclude the static IP address of the gateway in the Branch from allocation.

[H3C-dhcp-pool-br] quit

[H3C] dhcp server forbidden-ip 10.210.10.1

# Enable the detection of unauthorized DHCP servers.

[H3C] dhcp server detect

# Configure VLAN-interface100 to operate in the global address pool mode.

[H3C] interface Vlan-interface 100

[H3C-Vlan-interface100] dhcp select global

Note that:

After DHCP configuration is complete, IP addresses can be assigned to the workstations in the Branch only when a route is active between the HQ and the Branch.

III. Configuring the DHCP relay agent

This section mainly describes the DHCP server configuration. The following shows the basic DHCP relay agent configuration that ensures the DHCP relay agent to relay DHCP requests to the DHCP server. For details about DHCP relay agent configuration, see section 2.2  "DHCP Relay Agent/Snooping Configuration Examples".

<H3C> system-view

[H3C] dhcp-server 1 ip 10.214.10.1

[H3C] interface Vlan-interface 5

[H3C-Vlan-interface5] dhcp-server 1

2.2  DHCP Relay Agent/Snooping Configuration Examples

2.2.1  Network Requirements

A Cisco Catalyst 3745 switch is deployed in the HQ and serves as the DHCP server to assign IP addresses to the workstations in the Office branch. The branches are connected to an IRF (intelligent resilient framework) Fabric that serves as the central node and the DHCP relay agent to forward the DHCP requests from the workstations. Meanwhile, a lab DHCP server is used to assign IP addresses to the devices in the labs. The network requirements are as follows:

l           Configure the DHCP server in the HQ to assign the IP addresses in the 192.168.10.0/24 network segment to the workstations in the Office branch, with a lease period of 12 hours. Configure the IP addresses of the DNS server and WINS server as 192.169.100.2 and 192.168.100.3 respectively.

l           The IRF Fabric is connected to the branches and is comprised of four switches. It serves as the DHCP relay agent to forward the DHCP requests from the workstations in the Office and the devices in the labs. It is enabled to detect unauthorized DHCP servers.

l           An Ethernet switch in Lab1 serves as the Lab DHCP server to assign the IP addresses in the 192.168.17.0/24 network segment to the devices in Lab1, with a lease period of one day, and to assign the IP addresses in the 192.168.19.0/24 network segment to Lab2, with a lease period of two days. The lab DHCP server and the IRF Fabric are interconnected through the 172.16.2.4/30 network segment.

l           Configure the address checking function on the DHCP relay agent so that only the devices that are assigned legal IP addresses from the DHCP server are allowed to access the external network.

l           Configure address entry update on the DHCP relay agent so that it updates the address entries by sending requests to the DHCP server every one minute.

l           Enable DHCP snooping to support DHCP Option 82, adding local port information to the Option 82 field in DHCP messages.

l           Enable the DHCP relay agent to support DHCP Option 82 so that the DHCP relay agent keeps the original filed unchanged upon receiving DHCP messages carrying Option 82.

l           Enable the DHCP server to support DHCP Option 82 so that it assigns the IP addresses 192.168.10.2 through 192.168.10.25 to the DHCP clients connected to Ethernet1/0/11 on the DHCP snooping switch and assigns 192.168.10.100 through 192.168.10.150 to the DHCP clients connected to Ethernet1/0/12 of the DHCP snooping switch.

2.2.2  Network Diagram

Figure 2-2 Network diagram for DHCP relay agent/snooping integrated configuration

2.2.3  Configuration Procedure

In this example, the IRF Fabric is comprised of S3600 switches running software version Release 1510, a Quidway S3552 switch running software version Release 0028 is used as the DHCP snooping-capable switch, and a Quidway S3528 switch running software version Release 0028 is used as the Lab DHCP server.

For better readability:

l           The devices in the IRF Fabric are SwitchA, SwitchB, SwitchC, and SwitchD.

l           The DHCP snooping-capable device is referred to as “Snooping”.

l           The device serving as the Lab DHCP server is referred to as “LAB”.

I. Configuring IRF Fabric

The S3600 series support IRF Fabric. You can interconnect four devices to form a Fabric for centralized management of the devices in the Fabric. For details, see related sections in the operation manuals for the S3600 series.

II. Configuring the DHCP relay agent

Figure 2-3 Network diagram for DHCP relay agent configuration

Within the IRF Fabric, configuration made on a device can be synchronized to the other devices. Therefore, configuration is performed on Switch A only in this example.

# Configure to forward the DHCP requests from the Office to the DHCP server in the HQ.

<SwitchA> system-view

[SwitchA] dhcp-server 1 ip 192.168.0.3

[SwitchA] interface vlan-interface10

[SwitchA-Vlan-interface10] ip address 192.168.10.1 24

[SwitchA-Vlan-interface10] dhcp-server 1

# Configure to forward the DHCP requests from Lab2 to the Lab DHCP server.

[SwitchA-Vlan-interface10] quit

[SwitchA] dhcp-server 2 ip 192.168.17.1

[SwitchA] interface Vlan-interface 25

[SwitchA-Vlan-interface25] ip address 192.168.19.1 24

[SwitchA-Vlan-interface25] dhcp-server 2

# Configure the IP address of VLAN-interface17 as 172.16.2.5/30 for forwarding DHCP packets from the Lab DHCP Server to a non-local segment.

[SwitchA-Vlan-interface25] quit

[SwitchA] interface Vlan-interface 17

[SwitchA-Vlan-interface17] ip add 172.16.2.5 30

# Configure the address checking function on the DHCP relay agent. Make sure you configure the IP addresses and MAC addresses of the two DHCP servers as static entries for the security function.

[SwitchA-Vlan-interface17] quit

[SwitchA] dhcp-security static 192.168.0.3 000D-88F8-4E71

[SwitchA] dhcp-security static 192.168.17.1 0010-5ce9-1dea

[SwitchA] interface Vlan-interface 10

[SwitchA-Vlan-interface10] address-check enable

[SwitchA-Vlan-interface10] quit

[SwitchA] interface vlan-interface 25

[SwitchA-Vlan-interface25] address-check enable

[SwitchA-Vlan-interface25] quit

# Configure the address entry update interval on the DHCP relay agent.

[SwitchA] dhcp relay hand enable

[SwitchA] dhcp-security tracker 60

# Enable the DHCP relay agent to support DHCP Option 82 and adopt the strategy of keeping the original filed upon receiving DHCP messages carrying Option 82.

[SwitchA] dhcp relay information enable

[SwitchA] dhcp relay information strategy keep

# Enable the DHCP relay agent to detect unauthorized DHCP servers.

[SwitchA] dhcp-server detect

# Enable UDP-Helper so that the IRF Fabric can operate in the DHCP relay agent mode.

[SwitchA] udp-helper enable

# To ensure normal forwarding of DHCP packets across network segments, you need configure a routing protocol and advertise the network segments of interfaces. The following configuration uses RIP as an example. For the configuration of other routing protocols, see the parts covering routing protocols in product manuals.

[SwitchA] rip

[SwitchA-rip] network 192.168.10.0

[SwitchA-rip] network 192.168.19.0

[SwitchA-rip] network 172.16.0.0

 

&  Note:

For the DHCP relay agent using the IRF structure and the DHCP server in the HQ to communicate with each other, an active route must also be configured between them. This configuration is performed by the ISP or the user; therefore, it will not be covered in this document. 

 

III. Configuring the Lab DHCP server

Figure 2-4 Network diagram for the Lab DHCP server configuration

# Configure an address pool for Lab2 and specify the address range, lease period, and the gateway address.

<LAB> system-view

[LAB] dhcp enable

[LAB] dhcp server ip-pool lab2

[LAB-dhcp-lab2] network 192.168.19.0 255.255.255.0

[LAB-dhcp-lab2] expired day 2

[LAB-dhcp-lab2] gateway-list 192.168.19.1

# Configure the IP address of VLAN-interface17 as 172.16.2.6/30 and enable it to operate in global address pool mode.

[LAB-dhcp-lab2] quit

[LAB] interface Vlan-interface 17

[LAB-Vlan-interface17] ip address 172.16.2.6 30

[LAB-Vlan-interface17] dhcp select global

# Lab1 is connected to VLAN-interface15. Therefore, to assign the IP addresses in the 192.168.17.0/24 network segment to the devices in Lab1, you only need to configure VLAN-interface15 to operate in the interface address pool mode.

[LAB-Vlan-interface17] quit

[LAB] interface vlan-interface 15

[LAB-Vlan-interface15] ip address 192.168.17.1 24

[LAB-Vlan-interface15] dhcp select interface

[LAB-Vlan-interface15] quit

# To ensure that the lab DHCP server forwards DHCP packets normally, you need configure a routing protocol. The following configuration uses RIP as an example. For the configuration of other routing protocols, see the related parts in product manuals.

[LAB] rip

[LAB-rip] network 192.168.17.0

[LAB-rip] network 172.16.0.0

IV. Configuring DHCP snooping

Figure 2-5 Network diagram for DHCP snooping configuration

# Enable DHCP snooping and enable Option 82 support for DHCP snooping.

<Snooping> system-view

[Snooping] dhcp-snooping

[Snooping] dhcp-snooping information enable

[Snooping] dhcp-packet redirect Ethernet 0/11 to 0/13

V. Configuring the DHCP server in the HQ

# On the H3C series switches, port numbers, VLAN numbers, and the MAC addresses of the DHCP snooping device and the DHCP relay agent are added to DHCP Option 82. A complete piece of Option 82 information is a combination of the values of two suboptions:

Circuit ID suboption: It identifies the VLAN to which the clients belong and the port to which the DHCP snooping device is connected.

Figure 2-6 Packet structure of Circuit ID suboption

For example, the DHCP messages from clients connected to Ethernet1/0/11 are added with Option 82, whose Circuit ID suboption should be 0x010600040001000a, where 01060004 is a fixed value, 0001 indicates the access port’s VLAN is VLAN 1, and 000a is the absolute number of the port, which is 1 less than the actual port number, indicating the actual port is Ethernet1/0/11.

Remote ID suboption: It identifies the MAC address of the DHCP snooping device connected to the client.

Figure 2-7 Packet structure of Remote ID suboption

For example, the DHCP messages from clients connected to the DHCP snooping device with MAC 000f-e234-bc66 are added with Option 82, whose Remote ID suboption should be 02080006000fe234bc66, where 02080006 is a fixed value and 000fe234bc66 is the MAC address of the DHCP snooping device.

In this example, IP addresses are assigned based on port number only. Therefore, on the DHCP server, only a matching port number field in the Circuit ID suboption needs to be found.

 

&  Note:

The following configuration is performed on the Cisco Catalyst 3745 switch running IOS version 12.3(11)T2. If you are using any other models or devices running any other version, see the user manuals provided with the devices.

 

# Enable DHCP server and allocate IP addresses using Option 82 information.

Switch> enable

Switch(config)# configure terminal

Enter Configuration commands, one per line. End with CNTL/Z.

Switch(config)# service dhcp

Switch(config)# ip dhcp use class

# Create a DHCP class for the client connected to Ethernet1/0/11 of the DHCP snooping device and match the port number in the Circuit ID suboption of Option82, and replace the contents without match need with a wildcard "*".

Switch(config)# ip dhcp class office1

Switch(dhcp-class)# relay agent information hex 010600040001000a*

Switch(dhcp-class)# exit

# Configure a DHCP class for the client connected to Etherent1/0/12 of the DHCP snooping device and match the port number in the Circuit ID suboption of Option82.

Switch(config)# ip dhcp class office2

Switch(dhcp-class)# relay agent information hex 010600040001000b*

# Create an address pool for Office and specify address ranges for the two DHCP classes.

Switch(config)# ip dhcp pool office

Switch(dhcp-pool)# network 192.168.10.0

Switch(dhcp-pool)# class office1

Switch(dhcp-pool-class)# address range 192.168.10.2 192.168.10.25

Switch(dhcp-pool-class)# exit

Switch(dhcp-pool)# class office2

Switch(dhcp-pool-class)# address range 192.168.10.100 192.168.10.150

Switch(dhcp-pool-class)# exit

# Configure the lease period, gateway address, DNS server address, and WINS server address for the address pool.

Switch(dhcp-pool)# lease 0 12

Switch(dhcp-pool)# default-router 192.168.10.1

Switch(dhcp-pool)# dns-server 192.168.100.2

Switch(dhcp-pool)# netbios-name-server 192.168.100.3

After the above-mentioned configuration, the DHCP server can automatically assign an IP address, the gateway address, DNS server address, and the WINS server address for each device in Office.

2.3  Precautions

2.3.1  Cooperation Between DHCP Relay Agent and IRF

l           In an IRF network, the DHCP relay agent runs on all the units in the Fabric. But only the DHCP relay agent running on the master unit can receive and send packets to perform full DHCP relay agent functions. The DHCP relay agent running on a slave unit, however, only serves as a backup for the master unit.

l           DHCP is an application-layer protocol based on UDP. Once a slave unit receives a DHCP request, UDP-Helper redirects the packet to the master unit. Then, the DHCP relay agent running on the master unit gives a response back to the request and sends the real time information to each slave unit for backup. In this way, when the current master unit fails, one of the slaves becomes the new master and operates as the DHCP relay agent immediately. Therefore, make sure you enable UDP-Helper before using DHCP relay agent in an IRF system.

 


Chapter 3  Related Documents

3.1  Protocols and Standards

l           RFC2131: Dynamic Host Configuration Protocol

l           RFC2132: DHCP Options and BOOTP Vendor Extensions

l           RFC3046: DHCP Relay Agent Information Option