Quick Solution to nine common problems of network administrators (1)

Today's network is growing too fast. The key issue is that it also contains important applications and services of enterprises. Users and employees in the Organization need to work on the network to a large extent. In this environment, network technicians often need to diagnose complex network faults to keep the network running smoothly. This article discusses the symptoms, causes, and solutions of nine network problems that network technicians often encounter.

Problem 1: unable to obtain the IP address

Symptoms:The network looks unavailable. The operating system may warn that the IP address cannot be obtained from the DHCP server. After checking the NIC, the IP address is not obtained.

Cause:

1. the DHCP server may lack available IP addresses,

2. The DHCP Service on the server may be disabled,

3. The terminal device uses static IP addresses instead of Automatically Obtaining IP addresses.

4. DHCP requests from the terminal device are not sent to the server. This problem often occurs when a new device is configured for a VLAN. At this time, no VLAN is configured to forward DHCP requests to the DHCP server.

Solution:

The key question is, is this fault only affected by a single user or multiple users? If only one user is affected, check the NIC settings to ensure that it uses the DHCP service.

Next, check the vswitch and check whether the port and VLAN are configured. Check whether other devices on this VLAN can obtain the IP address. If these devices cannot obtain the IP address, the problem may be that the router did not forward the DHCP request to the DHCP server. If multiple devices on multiple sub-networks have this problem, the problem may be caused by the server itself. The server may not run the DHCP service, or it may not have enough IP addresses for allocation.

Problem 2: unable to connect to the Application Server

Symptoms:The application that the user tried to open issued a warning that the application server could not be connected. This often happens when you use an email server or a CRM application.

Cause:

This problem can be caused by multiple reasons. The key question is whether such a fault occurs frequently or occasionally. If the user has the correct IP address for this connection, there may be routing problems between the user and the server. Technicians can use a simple Ping command to confirm the problem. If the connection is occasionally dropped, the server may be too busy to respond to client requests.

Solution:

If it is not a routing problem, check the server load and resources, such as whether the server is busy running another task (such as backup )? If the server is not busy, check the network load between the client and the server. If there is a WAN connection, check the network load. In general, the client and the server are too high periodically. network applications may cause client connection problems. The best way to check is to use an SNMP tool to monitor the network utilization of these links. In addition, you must check the Ethernet errors on all vswitches and vrouters, which can cause packet loss between the client and the server.

Problem 3: Incorrect VLAN allocation

Symptoms:When installing new services (such as wireless or VoIP) on the network, VLAN is often used to isolate communication with other users. This requires configuring the correct VLAN that supports these service ports. If improperly configured, the service cannot run. The IP phone may not be registered in the Call Manager. The computer connected to the phone may not be connected to the key server. Wireless users may not be able to obtain the correct address in the wireless environment.

Cause:

The switch responsible for connecting these services may not be correctly configured. The switch does not communicate with the device in the unit, and no reconfiguration is performed on the switch to support these new services.

Solution:

Test which VLANs are supported by port verification. If possible, use a VLAN tag to generate specific VLAN communication and check which VLANs are configured on the port. Check the IP address provided by the DHCP server to determine which unlabeled VLAN is provided to the port. In addition, you should also check the configuration of the switch to verify the VLAN configuration.

Problem 4: Duplex Mismatch

Symptoms:If the duplex does not match, the connection is abnormal. The connection indicator on the vswitch and Nic is very active. The network performance is greatly reduced, and the throughput is reduced to kbps or lower.

Cause:One end of the Connection Works in Full Duplex (transfer and receive at the same time), and other devices work in half duplex (only transfer or receive at the same time ). The full-duplex end may interrupt the half-duplex end, resulting in an abnormal interruption of transmission at the half-duplex end. If the transmission is interrupted, you need to re-transmit the data frame. This greatly reduces the bandwidth available for half-duplex terminals.

Solution:

In almost all faults, the duplex mismatch is caused by the forced connection of one end (usually a switch) working in the full duplex, which causes the computer on the other end to automatically initiate a connection session. The problem is that automatic Session Initiation will force full duplex and match this configuration. But it is far from that simple. Forced to work at the full duplex end can no longer send the correct signal, which is exactly the signal that automatically initiates the connection session depends on the speed and duplex. During the connection, you need to guess the duplex status of the connection at the end of the connection session that is automatically initiated. In the case of uncertainty, the automatically initiated connection session will always work in half duplex. This is the mechanism of most duplex mismatch problems in the network. To do this, set all connections in the network to "Auto negotiation", as shown in:

Unless you have a clear reason not to do so. In these special cases, for example, in the mutual connection between switches, you must set both ends to full duplex.