H3C S3100-52P Manuale utente

Operation Manual – DLDP

H3C S3100-52P Ethernet Switch Table of Contents

Table of Contents

Chapter 1 DHCP Overview............................................................................................................ 1-1

1.1 Introduction to DHCP.........................................................................................................1-1

1.2 DHCP IP Address Assignment..........................................................................................1-1

1.2.1 IP Address Assignment Policy................................................................................1-1

1.2.2 Obtaining IP Addresses Dynamically...................................................................... 1-2

1.2.3 Updating IP Address Lease ....................................................................................1-2

1.3 DHCP Packet Format........................................................................................................1-3

1.4 Protocol Specification........................................................................................................1-4

Chapter 2 DHCP Snooping Configuration .................................................................................. 2-1

2.1 Introduction to DHCP Snooping.........................................................................................2-1

2.2 DHCP Snooping Configuration..........................................................................................2-2

2.3 Displaying DHCP Snooping...............................................................................................2-3

2.4 Configuration Example ......................................................................................................2-3

Chapter 3 DHCP/BOOTP Client Configuration........................................................................... 3-1

3.1 Introduction to DHCP Client...............................................................................................3-1

3.2 Introduction to BOOTP Client............................................................................................3-3

3.3 DHCP/BOOTP Client Configuration..................................................................................3-4

3.3.1 Prerequisites ...........................................................................................................3-4

3.3.2 Configuring a DHCP/BOOTP Client........................................................................3-4

3.3.3 Configuration Example............................................................................................3-5

3.4 Displaying the Information about a DHCP/BOOTP Client.................................................3-5

Operation Manual – DLDP

H3C S3100-52P Ethernet Switch Chapter 1 DHCP Overview

1-1

Chapter 1 DHCP Overview

1.1 Introduction to DHCP

With networks getting larger in size and morecomplicated in structure, lack of available

IP addresses becomes the common situation the network administrators have to face,

and network configuration becomes a tough task for the network administrators. With

the emerging of wireless networks and the using of laptops, the position change of

hosts and frequent change of IPaddresses also require new technology. Dynamic host

configuration protocol (DHCP) is developed in this background.

DHCP adopts a client/server model, where DHCP clients send requests to DHCP

servers for configuration parameters; and the DHCP servers return the corresponding

configuration information such as IP addresses to configure IP addresses dynamically.

A typical DHCP application includes one DHCP server and multiple clients (such as

PCs and laptops), as shown in Figure 1-1.

LAN

DHCP Server

DHCP Client DHCP Cl

DHCP Client DH

ient

CP Client

Figure 1-1 Typical DHCP application

1.2 DHCP IP Address Assignment

1.2.1 IP Address Assignment Policy

Currently, DHCP provides the following three IP address assignment policies to meet

the requirements of different clients:

zManual assignment. The administrator statically binds IP addresses to few clients

with special uses (such as WWW server). Then the DHCP server assigns these

fixed IP addresses to the clients.

zAutomatic assignment. The DHCP server assigns IP addresses to DHCP clients.

The IP addresses will be occupied by the DHCP clients permanently.

Operation Manual – DLDP

H3C S3100-52P Ethernet Switch Chapter 1 DHCP Overview

1-2

zDynamic assignment. The DHCPserver assigns IPaddresses to DHCP clients for

predetermined period of time. In this case, a DHCP client must apply for an IP

address again at the expiration of the period. This policy applies to most clients.

1.2.2 Obtaining IP Addresses Dynamically

ADHCP client undergoes the following four phases to dynamically obtain an IPaddress

from a DHCP server:

1) Discover: In this phase, the DHCP client tries to find a DHCP server by

broadcasting a DHCP-DISCOVER packet.

2) Offer: In this phase, the DHCP server offers an IP address. After the DHCP server

receives the DHCP-DISCOVER packet, it chooses an unassigned IP address

according to the priority order of IP address assignment and then sends the IP

address and other configuration information together in a DHCP-OFFER packet to

the DHCP client. The sending mode is decided by the flag filed in the

DHCP-DISCOVER packet, refer to section 1.3 "DHCP Packet Format” for detail.

3) Select: In this phase, the DHCP client selects an IP address. If more than one

DHCP server sends DHCP-OFFER packets to the DHCP client, the DHCP client

only accepts the DHCP-OFFER packet that first arrives, and then broadcasts a

DHCP-REQUEST packet containing the assigned IP address carried in the

DHCP-OFFER packet.

4) Acknowledge: In this phase, the DHCP servers acknowledge the IP address.

Upon receiving the DHCP-REQUEST packet, only the selected DHCP server

returns a DHCP-ACK packet to the DHCP client to confirm the assignment of the

IP address to the client, or returns a DHCP-NAK packet to refuse the assignment

of the IP address to the client. When the client receives the DHCP-ACK packet, it

broadcasts an ARP packet with the assigned IP address as the destination

address to detect the assigned IP address, and uses the IP address only if it does

not receive any response within a specified period.

Note:

The IP addresses offered by other DHCP servers but not used by the DHCP client are

still available to other clients.

1.2.3 Updating IP Address Lease

After a DHCP server dynamically assigns an IP address to a DHCP client, the IP

address keeps valid only within a specified lease time and will be reclaimed by the

DHCPserver when the lease expires. If the DHCPclient wants to usethe IPaddress for

a longer time, it must update the IP lease.

Operation Manual – DLDP

H3C S3100-52P Ethernet Switch Chapter 1 DHCP Overview

1-3

ags(2)

By default, a DHCP client updates its IP address lease automatically by unicasting a

DHCP-REQUEST packet to the DHCP server when half of the lease time elapses. The

DHCP server responds with a DHCP-ACK packet to notify the DHCP client of a new IP

lease if the server can assign the same IP address to the client. Otherwise, the DHCP

server responds with a DHCP-NAK packet to notify the DHCP client that the IPaddress

will be reclaimed when the lease time expires.

If the DHCP client fails to update its IP address lease when half of the lease time

elapses, it will update its IP address lease by broadcasting a DHCP-REQUEST packet

to the DHCP servers again when seven-eighths of the lease time elapses. The DHCP

server performs the same operations as those described above.

1.3 DHCP Packet Format

DHCP has eight types of packets. They have the same format, but the values of some

fields in the packets are different. The DHCP packet format is based on that of the

BOOTP packets. The following figure describes the packet format (the number in the

brackets indicates the field length, in bytes):

option(variable)

file(128)

sname(64)

chaddr(16)

giaddr(4)

siaddr(4)

yiaddr(4)

ciaddr(4)

flsecs(2)

xid(4)

hops(1)hlen(1)htype(1)op(1)

option(variable)

file(128)

sname(64)

chaddr(16)

giaddr(4)

siaddr(4)

yiaddr(4)

ciaddr(4)

flsecs(2)

xid(4)

ags(2)

hops(1)hlen(1)htype(1)op(1)

Figure 1-2 DHCP packet format

The fields are described as follows:

zop: Operation types of DHCP packets, 1 for request packets and 2 for response

packets.

zhtype, hlen: Hardware address type and length of the DHCP client.

zhops: Number of DHCP relays which a DHCP packet passes. For each DHCP

relay that the DHCP request packet passes, the field value increases by 1.

zxid: Random number that the client selects when it initiates a request. The number

is used to identify an address-requesting process.

zsecs: Elapsed time after the DHCP client initiates a DHCP request.

Operation Manual – DLDP

H3C S3100-52P Ethernet Switch Chapter 1 DHCP Overview

1-4

zflags: The first bit is the broadcast response flag bit. It is used to identify that the

DHCP response packet is sent in the unicast or broadcast mode. Other bits are

reserved.

zciaddr: IP address of a DHCP client.

zyiaddr: IP address that the DHCP server assigns to a client.

zsiaddr: IP address of the DHCP server.

zgiaddr: IP address of the first DHCP relay that the DHCP client passes after it sent

the request packet.

zchaddr: Hardware address of the DHCP client.

zsname: Name of the DHCP server.

zfile: Path and name of the boot configuration file that the DHCPserver specifies for

the DHCP client.

zoption: Optional variable-length fields, including packet type, valid lease time, IP

address of a DNS server, and IP address of the WINS server.

1.4 Protocol Specification

Protocol specifications related to DHCP include:

zRFC2131: Dynamic Host Configuration Protocol

zRFC2132: DHCP Options and BOOTP Vendor Extensions

zRFC1542: Clarifications and Extensions for the Bootstrap Protocol

Operation Manual – DLDP

H3C S3100-52P Ethernet Switch Chapter 2 DHCP Snooping Configuration

2-1

Chapter 2 DHCP Snooping Configuration

2.1 Introduction to DHCP Snooping

For the sake of security, the IP addresses used by online DHCP clients need to be

tracked for the administrator to verify the corresponding relationship between the IP

addresses the DHCP clients obtained from DHCP servers and the MAC addresses of

the DHCP clients.

zLayer 3 switches can track DHCP client IP addresses through DHCP relay.

zLayer 2 switches can track DHCPclient IPaddresses through the DHCPsnooping

function, which listens DHCP broadcast packets.

When an unauthorized DHCP server exists in the network, a DHCP client may obtains

an illegal IP address. To ensure that the DHCP clients obtain IP addresses from valid

DHCP servers, you can specify a port to be a trusted port or an untrusted port by the

DHCP snooping function.

zTrusted ports can be used to connect DHCP servers or ports of other switches.

Untrusted ports can be used to connect DHCP clients or networks.

zUntrusted ports drop the DHCP-ACK and DHCP-OFFER packets received from

DHCP servers. Trusted ports forward any received DHCP packets to ensure that

DHCP clients can obtain IP addresses from valid DHCP servers.

Figure 2-1 illustrates a typical network diagram for DHCP snooping application, where

Switch A is an S3100-52P series Ethernet switch.

Internet

DHCPclient

DHCP client

DHCPclient

Ethernet

DHCPclient

DHCPserver

Switch A (DHCP snooping) SwitchB (DHCPrelay)

Internet

EthernetEthernet

Internet

DHCPclient

DHCP client

DHCPclient

Ethernet

DHCPclient

DHCPserver

Switch A (DHCP snooping) SwitchB (DHCPrelay)

Internet

Ethernet

Internet

DHCPclient

DHCP client

DHCPclient

Ethernet

DHCPclient

DHCPserver

Switch A (DHCP snooping) SwitchB (DHCPrelay)

Internet

EthernetEthernetEthernet

Figure 2-1 Typical network diagram for DHCP snooping application

Figure 2-2 illustrates the interaction between a DHCP client and a DHCP server.

Operation Manual – DLDP

H3C S3100-52P Ethernet Switch Chapter 2 DHCP Snooping Configuration

2-2

DHCPclient

DHCPserver

DHCPclient

DHCPserver

DHCP-Request

DHCP-Offer

DHCP-ACK

DHCP-Renew

DHCP-ACK

DHCP-Discover

DHCP-Request

DHCP-Offer

DHCP-ACK

DHCP-Renew

DHCP-ACK

DHCP-Request

DHCP-Offer

DHCP-ACK

DHCP-Renew

DHCP-ACK

DHCP-Discover

DHCPclient

DHCPserver

DHCPclient

DHCPserver

DHCP-Request

DHCP-Offer

DHCP-ACK

DHCP-Renew

DHCP-ACK

DHCPclient

DHCPserver

DHCPclient

DHCPserver

DHCP-Request

DHCP-Offer

DHCP-ACK

DHCP-Renew

DHCP-ACK

DHCP-Discover

DHCP-Request

DHCP-Offer

DHCP-ACK

DHCP-Renew

DHCP-ACK

DHCP-Request

DHCP-Offer

DHCP-ACK

DHCP-Renew

DHCP-ACK

DHCP-Discover

Figure 2-2 Interaction between a DHCP client and a DHCP server

DHCP snooping listens the following two types of packets to retrieve the IP addresses

the DHCP clients obtain from DHCP servers and the MAC addresses of the DHCP

clients:

zDHCP-ACK packet

zDHCP-REQUEST packet

2.2 DHCP Snooping Configuration

Table 2-1 Configure the DHCP snooping function

Operation Command Description

Enter system view system-view —

Enable the DHCP

snooping function dhcp-snooping Required

By default, the DHCP

snooping function is disabled.

Enter Ethernet port

view interface interface-type

interface-number —

Operation Manual – DLDP

H3C S3100-52P Ethernet Switch Chapter 2 DHCP Snooping Configuration

2-3

Operation Command Description

Set the port

connected to a

DHCP server to a

trusted port

dhcp-snooping trust Optional

By default, all ports of a

switch are untrusted ports.

Note:

When you need to enable DHCP snooping on the switches in a fabric state, configure

the fabric ports on all devices to be trusted ports to ensure that the users connected to

each device can obtain IP addresses.

2.3 Displaying DHCP Snooping

After the above configurations, you can verify the configurations by executing the

display command in any view.

Table 2-2 Display DHCP snooping

Operation Command Description

Display the user IP-MAC address

mapping entries recorded by the

DHCP snooping function

display

dhcp-snooping [unit

unit-id ]

Display the (enabled/disabled)

state of the DHCP snooping

function and the trusted ports

display

dhcp-snooping trust

You can execute the

display command in

any view

2.4 Configuration Example

I. Network requirements

As shown in Figure 2-1, the Ethernet1/0/1 port of Switch A ( S3100-52P ) is connected

to Switch B (acting as a DHCP relay). A network segment containing some DHCP

clients is connected to the Ethernet1/0/2 port of SwitchA.

zEnable the DHCP snooping function on SwitchA.

zSet the Ethernet1/0/1 port of Switch A to a trusted port.

II. Configuration procedure

# Enter system view.

<H3C> system-view

# Enable the DHCP snooping function.

Operation Manual – DLDP

H3C S3100-52P Ethernet Switch Chapter 2 DHCP Snooping Configuration

2-4

[H3C] dhcp-snooping

# Enter Ethernet1/0/1 port view.

[H3C] interface Ethernet1/0/1

# Set the port to a trusted port.

[H3C-Ethernet1/0/1] dhcp-snooping trust

Operation Manual – DLDP

H3C S3100-52P Ethernet Switch Chapter 3 DHCP/BOOTP Client Configuration

3-1

Chapter 3 DHCP/BOOTP Client Configuration

3.1 Introduction to DHCP Client

As the network scale expands and the network complexity increases, the network

configurations become more and more complex accordingly. It is usually the case that

the computer locations change (such as the portable computers in wireless networks)

or the number of the computers exceeds that of the available IP addresses. The

dynamic host configuration protocol (DHCP) is developed to meet these requirements.

DHCP adopts the client/server model, where DHCP clients request DHCP servers

dynamically for configuration information, and the DHCP servers in turn return

corresponding configuration information based on policies.

A typical DHCP implementation usually involves a DHCP server and multiple clients

(such as PCs and portable computers), as shown in Figure 3-1.

LAN

DHCP Server

DHCP Client DHCP Client

Figure 3-1 A typical DHCP implementation

The interactions between a DHCP client and a DHCP server are shown in Figure 3-2.

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