Technical service interview review

The first interview was held at 10.50 this morning. The interview position was technical service. I discussed many technical issues with the technicians. It was still necessary to conduct a review. During the interview, I felt nervous and lost my mind. There is a blank and the basic knowledge is not solid. Next, we will review the problem:

ARP protocol

• Q: What is the ARP protocol for?
• A: Well~ In my opinion, ARP is IP-to-MAC conversion, because the transmission of data packets must be implemented on the MAC address, and the MAC address is required during the transmission process. If you want to go to a certain destination, you must implement it. On top of the MAC address, the OSI7 layer model falls down layer by layer, and the header needs to be added, um~ I basically understand it this way.

Review: ARP is an address resolution protocol, and its corresponding MAC address is obtained according to the IP address that has been reached. In the OSI7-layer model, each layer from top to bottom must encapsulate the header. After the header of the network layer (including source IP and destination IP) is encapsulated, the layer 2 header (including source MAC and destination MAC) must be encapsulated. ), but here comes the problem, if you know your own MAC, what about the MAC address of the peer? At this time, there is a protocol called ARP protocol, which works like this: When it is necessary to obtain the MAC address of the peer (assuming that they are all in a broadcast domain), an ARP Request broadcast message will be sent first, and the message contains the MAC address of the peer. IP address, when the peer receives an ARP broadcast message containing its own IP address, it will first reply with an ARP response message, which contains its own IP and MAC address, and will also record the peer’s MAC address. This is the content in the ARP Request message:

the destination MAC is all F, indicating the broadcast address, that is, broadcast transmission.

• Q: What is the use of free ARP?
• A: Gratuitous ARP is used to check whether there will be an address conflict. When a terminal accesses the network, it will first send a gratuitous ARP. If there is a reply packet, it proves that there is already a device using this address in the network, so you can Check whether the address conflicts.
• Q: If an address conflict is found, which of the two computers can access the Internet?
• A: Well, maybe intermittently. (I'm not sure here, it's vague)
• Q: Have you experimented in the school computer room?
• A: It seems to be, because there seems to be an address conflict before.
• Q: I suggest you go back and try, because only one device can access the Internet.

Replay: After the device is connected to the network, it will send an ARP Request message to request its own IP address. If it receives a response, it proves that a device in the network has already used this IP address, and this device cannot Go online. (After asking the first question, I feel okay psychologically, because I have already answered all the questions in sevens and eighties)

Address classification and scope

• Q: Tell me about the private addresses in Class A addresses.
• A: ..., private address? Class A? (Inner OS: Isn’t class A a shared address? Where did the private address come from? But you can’t be cold, let’s talk about everything) I don’t remember this clearly, but I know that class A starts with 0, and class B starts with 10. Class C 110, Class D 1110.
• Q: Then tell me about the range of Class A addresses.
• A: ... (It's over, I really don't remember) I kind of forgot. (Finally the interviewer told me the answer)

Replay:
Class A address range: 1.0.0.1-127.255.255.254, private address: 10.XXX, reserved address: 127.XXX
Class B address range: 128.0.0.1-191.255.255.255 private address: 172.16.0.0-172.31. 255.255 Reserved address: 169.254.XX (this type of address will be obtained when the address cannot be obtained using DHCP)
Class C address range: 192.0.0.1-223.255.255.254 Private address: 192.168.XX

• Q: How many subnets can the network be divided into by 192.168.1.0/25?
• A: Two. (The first feeling at the time was 25=24+1, so the 25th bit can be selected, which is to choose from 1 and 0 respectively)
• Q: Tell me about the scope of the subnet.
• A: Hmm~192.168.0.0 to hmm... (it's very confusing at this time, I don't know what I'm talking about) that is, the 25th bit can be changed, which are 0 and 1 respectively (it seems a bit confusing again here) 192.168. 0.0 to... (already starting to gibberish)
• Q: Why don't you look at what you are talking about (it's a bit embarrassing at this time) you think, divide it into two subnets, the first range is 192.168.1.1-192.168.1.126, 192.168.1.127-192.168.1.254, why One and last address not working?
• A: The first address is the network number, and the last one is the broadcast address. (Okay no more mistakes)

VLAN

• Q: What is VLAN?
• A: VLAN is a virtual local area network, which is used to isolate broadcast domains. Devices in different broadcast domains cannot communicate. For example, the financial department and the sales department belong to two different VLANs, and then access restrictions can be realized, and it is also important for security. Sex has improved.
• Q: What is the most important function?
• A: (I thought what I said was wrong, so I said something else) VLAN can restrict communication between different users, and devices in different VLANs cannot communicate with each other, but communication between different devices in VLAN can also be set.
• Q: The most important thing is to isolate broadcast domains to prevent broadcast storms.

switch loop

• Q: Have you ever seen a switch loop?
• A: I have seen it, that is, the switches continue to forward data packets, then consume resources, and finally transmit infinitely, and the cycle goes back and forth.
• Q: Can the device still communicate after the loop?
• A: It should be ok. (Because I'm not sure, I think it's just too much resource consumption, but communication is still possible)
• Q: No, after a loop occurs, you can test it. It is quite normal to lose 90 out of 100 data packets. (I haven't touched the real phone, so I suffered a lot)
• A: Oh oh...
• Q: What techniques can be used to prevent it?
• A: (I will) you can use STP and other technologies, RSTP or MSTP are both available. The convergence speed of STP is too slow. Compared with the direct connection failure, wait 20s longer for the BPDU aging time.
• Q: OK.

protocol priority

• Q: What is the priority of OSPF?
• A: (Not sure here) It should be 60.
• Q: What about static routing?
• A: It seems to be 0.
• Q: What about the default route?
• A: (I have been confused about default and static before, and I didn’t know which size is better)
• Q: Which one is better?
• A: (began to talk nonsense)
• Q: Serious nonsense. (and start explaining to me)

Review:
OSPF priority 10, RIP priority 100, static route priority 60, direct route 0, default route (the last choice) 255, the lower the priority, the better.

VPN

• Q: Where is MPLS VPN widely used?
• A: Among operators, it is widely used across domains.
• Q: Which layer is MPLS at?
• A: Between layer 2 and layer 3, it is a layer 2.5 protocol.
• Q: Tell me about IP SecVPN.
• A: (I really forgot about this one) Hmm~ IP SecVPN has a security authentication function. I remember there is an AH authentication, but I am not familiar with the others.
• Q: Okay then.

protocol model

• Q: How many layers does the TCP/IP model have?
• A: 4 layers.
• Q: The four floors?
• A: (I made a mistake here, but someone happened to talk to the interviewer, so I just missed it)

Review: TCPIP is divided into 4 layers, namely application layer, transport layer, network layer, and network interface layer.

NAT protocol

• Q: What is the NAT protocol used for?
• A: The NAT protocol is used to solve the shortage of IP addresses. The shared IP addresses have been allocated in 2001. Tablets, computers, mobile phones, and smart homes all need IP addresses. If only public addresses are used, it will not be enough. up. Using private addresses allows addresses in one area to use one public address, thereby alleviating the pressure on IP addresses.

Telnet and SSH

• Q: Do you know about SSH?
• A: I know. SSH adds an additional secure shell, just like the S of HTTPS is SSH, which is more secure than telnet.
• Q: Where is the safety?
• A: Telnet uses plaintext for transmission, and SSH uses encrypted data for transmission, which is more secure.
• Q: OK.

Layer 2 protocol

• Q: Do you know the L2TP protocol?
• A: (Confused) Is it PPP?
• Q: Then tell me about PPP.
• A: Under the PPP protocol, there are PAP, CHAP and other authentication methods, which are more secure. In addition, PPPOE realizes the function of charging and timing.
• Q: Hmm.

Without a trial, without gaining wisdom, I am very grateful to the interviewer today, who made me see my own shortcomings, and hope that I can become even better in the future.