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Network Security Learning (XIII) data link layer

2022-07-25 22:51:00 【Shake light·】

Data link layer （2 layer Date link layer）

1. Belongs to the second floor

2. The transmission unit ： frame

3. Frame structure ： Pictured

Frame head ：6+6+2=14 byte

MTU value ：1500 byte

Frame tail ：4 byte

Frame header content ： The goal is mac, Source mac, type

Role of type ： Identify the upper layer protocol

0x0800： The upper level is IP agreement

0x0806： The upper level is ARP agreement

The frame format ：

802.3（ Wired network card ）

802.3u Achieve 100m Fast Ethernet

802.3z Optical fiber realizes gigabit network

802.3ab The twisted pair realizes Gigabit Ethernet

802.3ae Achieve 10 trillion

802.3ba Achieve 100000 trillion

802.11（ wireless network adapter ）

802.11a The highest speed 54Mbps（5g）

802.11b The highest speed 11Mbs （2.4g）

802.11g The highest speed 54Mbs（2.4g）

802.11n The highest speed 600Mbs

mac Address 12 position 16 Base number = 16*4=48 Bit binary = 48 byte /8 = 6 byte

MTU The value is only 1500

4. Equipment working at the data link layer ： Switch / network card

5. How switches work ：

After receiving a data frame ：

First, learn the source in the frame MAC Address to form MAC Address table

Then check the target in the frame MAC Address , And match MAC Address table

If there is a match in the table , Unicast forwarding

If there is no match in the table , In addition to the receiving port, broadcast forwarding

MAC The aging time of the address table is... By default 300 second （ Modifiable ）

Port status ：up/down

down Three possibilities ：

1） artificial down fall

2） Rate mismatch

3） Duplex mode mismatch （ Duplex duplex）

Duplex mode ： Simplex , Half duplex , full duplex

Basic working mode and commands of the switch

Configure the network device for the first time , Need to use console Line

stay PC Need to use “ hyper terminal ” Or other third-party software

1） User mode

switch>

You can view the basic and simple information of the switch , And the configuration cannot be modified

2） Privilege mode ：

Input switch>enable

switch#

You can view all configurations , And the configuration cannot be modified , But you can test , preservation , Initialization and other operations

3） Global configuration mode

Input switch#configure terminal

switch（config）#

Configuration cannot be viewed by default ！ You can modify the configuration , And the global effect ！

4-1） Interface configuration mode

Input switch（config）# interface f0/1

switch（config-if）#

Input switch（config）#line console 0 Get into console mouth / Line / Console mode

Configuration cannot be viewed by default ！ You can modify the configuration , And effective for this port ！

4-2）console mouth / Line / Console mode

Input switch（config）#line console 0

Configuration cannot be viewed by default ！ You can modify the configuration , And right console Entry into force ！

command
exit Exit level 1
end Go straight back to privileged mode

configure teriminal
hostname name Configure host name （ Interface configuration mode ）

line console 0
password password
login Set user password （ Console mode ）

configure teriminal
enable password Set a privileged password （ Interface configuration mode ）
enable secret password Encrypt privileged password （ If plaintext password has been configured , Clear text password will be invalid ）

Support command abbreviations

？ View command

History commands

tab Completion order

Shortcut key

ctrl+u Delete a line
ctrl+a Jump to the beginning of the line
ctrl+e Jump to the end of the line

Do not save after configuring the switch , After restart, the configuration is gone

Because there is a file in memory

running-config

The first time I turn on the computer , The system will automatically create a new clean... In memory running-config

Save configuration

Under privilege mode ：

copy running-config startup-config

write

Switch restart ：

en
reload

Switch startup action ：

First go to the hard disk to find startup-config Whether there is

If it doesn't exist , Create a new... In memory run

If there is , Then it is copied into memory and named running-config

The interface rate of the switch

int f0/0
speed rate

see running-config To configure

en
show running-config
sh run Abbreviation

Configure duplex mode

int f0/1
duplex half Half duplex
full full duplex
auto The adaptive

see stratup-config To configure

en
show stratup-config

see MAC Address table

en
show mac-address-table

View the brief information of the interface

en
show ip interface brife
show ip int b Abbreviation

Close the interface manually

int f0/x

shutdown Turn on

no shutdown close

Commands in privileged mode can be enforced in other modes

do xxx

But no code completion

Delete the configuration

1） Delete it wherever you configure it

2） Add before configuration no

3） The original command has parameters , Delete unnecessary parameters

no shutdown

Empty / erase / Initialize configuration

erase startup-config

Configure for layer 3 ports IP Address ：

int f0/0
ip address 10.1.1.254 255.255.255.0
Abbreviation ip add 10.1.1.254 255.255.255.0
no shutdown
exit

Remote control router

Input ：
Router(config)#line vty 0 ? Enter the virtual terminal
？ Take 0-15, Represents the number of people with the most remote control
You need to set the password after entering ( The setting method is the same as user password )
Router(config-line)#password xxxxxx
Router(config-line)#login
You can also set no password
Router(config-line)#login local Use local Do Authentication
To configure local（ Local database ）
Router(config）#username xx password xxx
And it can only be accessed when the privileged password is set

Default on 23 The port number can be passed through telnet visit

Get into Virtual terminal after , You can also change the port number , Use ssh Connect

transport input ?
all Turn on 22 23 port
none Close all ports
ssh open 22 port
telnet open 23 port

But before setting, you need to turn on the encryption algorithm

Router(config）# crypto key generate rsa Generate the key
Then you will be asked to enter the length of the key pair

But to execute the above command, you need to configure the domain name and router name （ This domain name has nothing to do with the company's domain name ）