HCIA assessment

1. describe TCP Three times mobile phone system
answer ： The first handshake
The client sends a request packet to the server , Request to establish a connection with the server （ Explicit confirmation ）

The second handshake
The server sends back a confirmation and request package to the client , Tell the client that I have received your request , And ask to establish a connection with you （ Explicit confirmation ）

The third handshake
The client sends a message to the server again ack package , Means I received your request （ Implicit confirmation ）

2. detailed DHCP working principle
answer ： because dhcp use udp encapsulation , So in order to ensure reliable transmission , The acknowledgement retransmission mechanism is adopted .

First step ： The client sends a... To the server DHCP discover package （ Broadcast packet ）： To discover dhcp Server and request ip Address .
Source port number 68, Target port number 67; Source IP0.0.0.0, The goal is IP255.255.255.255; Source MAC For the host MAC, The goal is MAC by FF-FF-FF-FF-FF-FF.

The second step ： Server received discover Send a packet to the host DHCP offer package （ Broadcast packet ）： Include host mac Address 、 The server assigns to the host IP、 Mask 、 gateway 、dns The content such as .
Source port 67, Target port 68; Source IP For the server IP, The goal is IP by 255.255.255.255; Source MAC For the server MAC, The goal is MAC by FF-FF-FF-FF-FF-FF.

The third step ： The host sends a message to the server again DHCP request package （ Broadcast packet ）： Contains the server assigned to the host IP、 Mask 、 gateway 、dns The content such as . To tell the server , The host decides to use the IP Address , Mask , Gateway, etc. .
Source port number 68, Target port number 67; Source IP0.0.0.0, The goal is IP255.255.255.255; Source MAC For the host MAC, The goal is MAC by FF-FF-FF-FF-FF-FF.

Step four ： The server sends a message to the host DHCP ACK package （ Broadcast packet ）： Tell the host that you can use the IP, Mask , Gateway, etc. .
Source port 67, Target port 68; Source IP For the server IP, The goal is IP by 255.255.255.255; Source MAC For the server MAC, The goal is MAC by FF-FF-FF-FF-FF-FF.

3. take 16.16.16.0/22 Divided into 8 Subnet
answer ：
0001 0000.0001 0000.0001 0000.0000 0000
1111 1111.1111 1111.1111 1100.0000 0000

①0001 0000.0001 0000.0001 0000.0000 0000
16.16.16.0/25
②0001 0000.0001 0000.0001 0000.1000 0000
16.16.16.128/25
③0001 0000.0001 0000.0001 0001.0000 0000
16.16.17.0/25
④0001 0000.0001 0000.0001 0001.1000 0000
16.16.17.128/25
⑤0001 0000.0001 0000.0001 0010.0000 0000
16.16.18.0/25
⑥0001 0000.0001 0000.0001 0010.1000 0000
16.16.18.128/25
⑦0001 0000.0001 0000.0001 0011.0000 0000
16.16.19.0/25
⑧0001 0000.0001 0000.0001 0011.1000 0000
16.16.19.128/25

4. sketch OSI In the model 2 3 4 Layer action
answer ：4 layer ： Transport layer , Distinguish traffic , Define the data transmission mode （ There are mainly TCP,UDP Two protocols ）
3 layer ： The network layer , Addressing , Addressing
2 layer ： Data link layer , According to the transmission medium of the physical layer, the communication protocol is used to control the transmission of these data .

5. What is the horizontal segmentation mechanism ？
answer ： The horizontal partition mechanism is used in link state dynamic routing protocols , To prevent routing loops . A route received by an interface , It can no longer be forwarded through this interface .

6.RIP What are the timers of the protocol ？ What do they mean respectively ？
answer ：
Update the timer ： It is used to realize periodic interaction of routing items （ Whether there is data flow ）, The default is 30 Update the information in the routing table every second .【 To prevent update synchronization ,RIP Will 15% Error sending update , That is to say, the range of the actual cycle of sending updates is 25.5-30 second .】

Failure timer ： If a certain route information is in 180s No updates or confirmations have been received within , Then this route information will be marked as “ invalid ”.【 At this time, it is not really invalid , The prompt is about to expire , This route will not be deleted from the routing table immediately , But will wait 60s Time will really delete .】

Inhibit timer ： Expiration timer expires , Enter immediately 180s Suppression timer , Used to prevent routing loops （ It's also 180s）. If a small number of route update packets are received from any adjacent router during suppression , Then restore the network and delete the suppression timer ; If the number of hops of a route update received from an adjacent router during suppression is greater than the number of hops recorded in the routing table , The update will be ignored .

Refresh timer ： If the routing entry is in 240 Seconds has not received the update , Delete this route entry from the route table .

7. Describe the types of all ports of the switch ？
answer ：
Access： Access interface , Only belong to one VLAN Trunk： Relay interface ,trunk The link can transmit multiple different messages at the same time VLAN
Hybrid： Hybrid interface

8. What is flooding （ flooding ） Mechanism ？ Describe the working process
answer ：
The switch receives a data frame that needs to be flooded , The data frame will be forwarded through all interfaces except the receiving data interface .
Default flooding data frame ：1. radio broadcast FFFF.FFFF.FFFF 2. Multicast （01-00-5E-）3. Unknown unicast

9. What is priority 、 Cost value ？ Their functions are ？
answer ：AD value （priority priority ）： Manage distance 0-255, The smaller the numerical , The more credible .（ The smaller the better ）, The default priority values of routing entries generated by different routing methods are different .
metric value （cost Cost value ）： measurements , Calculation of different routing methods metric The way the value works is not the same , The smaller the better .

effect ： To find the best link in the network

10. Describe... In a switched network trunk Interface features
answer ： Relay interface ,trunk The link can transmit multiple different messages at the same time VLAN