10 problems that network administrators often encounter

Source: Internet
Author: User
Tags get ip requires switches

The rapid development of the current network, and host a number of important business applications and services, but also more and more network users. In this network environment, in addition to the network engineers to complete such as the simple task of adding devices to the network, more often they need to deal with and solve some complex network problems, to ensure the normal and fast operation of the network.

This white paper discusses the 10 network problems that technicians often encounter, and describes their behavior, causes, and solutions.

Problem-I port cannot connect to network

Phenomenon: Plug the computer, phone, wireless access point or printer into the network jack on the wall, and the network connection is not normal. The switch port connection LEDs and the NIC connection LEDs are not lit.

Reason: If the network connection on the wall jack is not repaired, there is often a problem of dropping or not connecting. In many enterprises, only those connections that are frequently used are repaired. When the office or conference room is moved, it is sometimes found that the unused network jacks have not been tested, or that the jacks that could not be connected may have been caused by a registration error. In addition, the switch port may be forcibly closed.

Workaround: Check and verify that the switch port has been activated and that the network connection has been repaired. When any device is moved to the office, be sure to test the new network connections to make sure they are working properly. As far as IP telephony is concerned, there may be a lack of power supply for the phone.

Problem 2– Unable to get IP address

Phenomenon: Network paralysis or failure of the normal operation. The operating system may indicate that the client is currently unable to obtain an IP address from the DHCP server. After checking the status of the NIC, it was found that no IP address was assigned.

Reason: The IP address assigned from the DHCP server was not received. The DHCP server's IP address is exhausted, the server's service is paralyzed, and the terminal device may be configured to use a static IP address instead of a DHCP allocation, and the terminal device's DHCP request never arrives at the server side, which can cause the client to fail to obtain the IP address.

Especially if a new device is configured with a virtual local area network (VLAN), the device will not be able to obtain an IP address without establishing a service request connection to the server. When a new device is configured to a VLAN, if a DHCP request is not relayed to the DHCP server, the request cannot be sent to the DHCP server side.

Solution: The key question is how many users have the same problem, one user or multiple users? If only one user is affected, verify that the client's network settings are configured to use Dynamic Host Configuration Protocol (DHCP).

Next, check which vlan the switch's ports are divided into, and check to see if other devices belonging to the VLAN can obtain an IP address. If they also cannot obtain an IP address, the problem may be that the router did not forward DHCP requests to the DHCP server. If this problem occurs with many devices in multiple subnets, there may be a problem with the DHCP server, that is, the server's DHCP service may not be running, or its IP address is exhausted.

Problem 3– cannot connect to the application server

Behavior: When a user attempts to open an application, the system may be prompted not to connect to the application server. This problem may occur when using e-mail applications or customer relationship management (CRM) applications. The common complaint that is reflected to the help desk is that the network is paralyzed, and this may not be the real cause.

Cause: Many reasons can cause clients to not be able to connect to the application server. The key is to ask the user whether the problem occurs frequently or intermittently? If the user terminal has acquired a correct IP address, there may be a problem with the routing between the user and the server. In this case, you can verify it with a simple ping test. If the connection is intermittent, it may be because the server is too busy to respond to client requests in a timely manner.

Workaround: If you find that routing is not a problem after using the ping test, check your server's load and resources. Check that the server is busy performing a task such as backup? If this is not the case, check the network load between the client and the server and focus on the WAN connection (if any).

Typically, high network utilization between the client and server can also cause clients to connect to the application server. The best approach is to use an SNMP tool to monitor network usage on these links. Also, look for Ethernet errors on all switches and routers that can cause data packets to be lost between the client and the server.

Problem 4–vlan Configuration Error

Phenomenon: When creating new services on the network, such as wireless broadband or IP telephony, you typically use VLANs to isolate traffic between them and other users. This requires that the switch port that hosts these services be configured with the correct VLAN. If the VLAN is not configured correctly, the service may not function correctly. If the IP phone is not authorized by a call management device, the personal computer connected to the phone may not be able to connect to the critical server, or the wireless user may not have the correct IP address for the wireless environment.

Reason: The switch that hosts these service connections is not properly configured. Perhaps there is no communication within the company to configure which ports to support the new service.

Workaround: Test to verify which VLAN the port supports. If conditions permit, use a VLAN tag to generate a business for a particular VLAN, and check to see if the VLAN is configured on this port. Determine which ports the VLAN is configured on by checking the IP addresses provided by the DHCP server. Also, check the configuration of the switch to verify the VLAN configuration.

Problem 5– Duplex mode mismatch

Phenomenon: Network can connect, and duplex mode does not match, can cause network performance is very poor. In this case, both the switch and the network card's link light are on. The performance of the network is greatly affected, the throughput will be reduced to 100Kbps or lower.

Cause: The network-attached side of the device works in full duplex mode, which can send and receive data simultaneously, while the other side of the device works at Half-duplex (only send or receive at the same time) mode. Full duplex side of the device does not need to wait to send data continuously, whether the other party can receive data it will send.

The half duplex side device must wait until it is sent before it can start sending. This means that the Full-duplex side of the device may interrupt the Half-duplex side of the data transmission, resulting in half-duplex lateral abort transmission. If the transfer is aborted, you will need to retransmit the data frame. This greatly reduces the bandwidth available to half duplex side devices.

Solution: In general, if the network of devices on both sides of the duplex does not match each other, the common way to take the network connected to the side of the device (usually the switch) is forced to configure Full-duplex, while the other side of the equipment (such as personal computer) configured to adaptive Network link state.

Ideally, the adaptive feature will be able to confirm each other's Full-duplex settings and match this link setting. But that is not necessarily the case. Devices that are forced to be set to Full-duplex mode no longer send the correct signal. And the device on the other side of the network just needs these signals to determine the speed and duplex of the link, as well as the adaptive link settings.

Therefore, in this case, the device that needs the adaptive link has to guess how the link is duplex. In the event that the duplex mode is not confirmed, the adaptive function will have the default link state of Half-duplex. This is the main reason why the network does not match the duplex mode in most cases. To solve this problem, you need to set all the connections on your network to be adaptive-unless you do have other reasons. In these events, such as connections between switches, be sure to set both devices to Full-duplex.

Problem 6– Slow Application performance

Behavior: The application is running slowly. It may freeze on an interface or stop running while accessing data. In general, the Poor network link status is the main cause of these problems.

Reason: Determining performance issues for network applications can help server maintenance personnel locate problem sources. Many problems can cause applications to run slowly. The most common causes are backup operations at the server's production time, slow response from the database server, and serious loss of packets in the network. For a network technician, the most important thing is to first determine whether the server or the network caused a slow response.

To determine this, you can capture traffic from a client. Look for any retransmission packets between the client and the server. If there is a retransmission phenomenon, it means that there is a packet loss during network transport, which can severely affect the performance of the application. If there is no retransmission between the client and the server, the problem may be on the server, in which case the server can be strictly monitored.

Solution: In tracking a fault, although the use of Packet Analyzer is more difficult, they are often equipped with simple counters, can display TCP retransmission. Use this counter to help us determine whether there is a packet loss in the network between the client and the server.

Look for Ethernet errors in any switch and router between the client and server (frame detection sequence errors, calibration errors, or collisions) that can cause packet loss. If there are no errors, observe whether there is a packet loss caused by high link utilization on the WAN.

Problem 7– Printing failure

Phenomenon: Sometimes there are printing failures. The printer may not be a problem, and the task sent to the printer may not function properly.

Reason: First you need to determine if only one user is experiencing the problem, or if several users are experiencing the same problem. If only one user encounters it, it may be that his computer is not properly connected to the print server. If this is not the case, there may be a problem with the network connection between the client and the printer. Packet loss may cause printing problems, and network connectivity failures on the printer may also cause printing problems.

Workaround: Check the configuration of the printer, make sure it has a correct IP address, and can connect to the print server (if the print server and printer are separated). Sometimes, updating the printer driver can also solve the printing problem. Overall, you need to make sure that your printer is up to the network and that all printer drivers are up to date.

Problem 8– Cable Failure

Phenomenon: Customer's computer terminals can connect to the network, the network performance is very poor. The computer terminal may not be able to connect to the network at all.

Why: In today's network, gigabit links to desktop systems are common. The gigabit link requires four pairs of cables, so any cable with a performance below the 5-class line cannot support gigabit connections. For older buildings, this problem must be taken into account. In addition, any number of cables if the twist (usually near the RJ-45 port or wiring board) may cause loss of signal. This will result in a frame detection sequence error (FCS) on the switch port or NIC.

Workaround: When there is a network failure due to a cable problem, the cable needs to be replaced in most cases. If the cable is caused by no twist, then twisting the cable can generally solve the problem. When you need to host new technologies such as Gigabit link or Ethernet, you must use a Class 5 cable or a better cable.

Problem 9–dns Failure

Phenomenon: Users cannot access the Internet or critical applications. Maybe the network is paralyzed.

Cause: The problem may be attributed to a domain Name Service (DNS) failure. The client cannot resolve the name of the server based on the IP address of the server, so the connection request cannot be sent. This problem is usually caused by misconfigured DNS servers on the client, where DNS requests sent by clients do not belong to the server's database, or packets are lost on the network. DNS is a UDP based protocol, so missing packets will not be retransmission, causing DNS failures.

Workaround: Check the DNS server for client configuration and settings. If the server is misconfigured, change the server that is set up on the client or adjust the configuration on the server that provides the domain name service.

Repeatedly test the connection between the DNS server and the client to confirm whether there is a response delay due to packet loss. If a packet is missing, check the Ethernet error between the client and the server. Capture a failed DNS request to confirm that there is a DNS response from the server. It is best to be able to start a tool that can continuously test the DNS server and generate alarms when problems arise.

Problem 10– client cannot connect to wireless network

Symptom (s): The client can detect the wireless access point, but cannot access the wireless network.

Reason: Security authentication, wireless channel interference and signal blind area may cause this problem. Because wireless signals are not visible, it is difficult to trace these problems without the help of a professional wireless tool.

Solution: Use a wireless monitoring tool to measure the signal strength of the affected area and, if possible, carry out a field survey to identify a malicious or unknown access point in the area. May be due to the wireless channel overlap and generate interference signals, thus affecting the access to wireless network user perception.

Check for interference signals from other access points around, or noise generated by other interfering devices such as microwaves and cordless phones. Monitor the access point that the client is attempting to connect to, and pay close attention to the step in which the connection fails--association, authentication, or authorization.


We list 10 problems that technicians are likely to encounter in the current network. In most cases, you can position the problem on one or more things and quickly resolve the problem.

Write down the solutions to common problems and share them with other technicians, which can also solve the problem quickly. If you have the right tools, you can quickly and easily locate and solve problems, restore the normal operation of the network in time.

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