How to troubleshoot network connectivity problems (325487)

MORE INFORMATION

Network connectivity problems have various causes but typically occur because of incorrect network adapters, incorrect switch settings, faulty hardware, or driver issues. Some connectivity symptoms are intermittent and do not clearly point to any one of these causes.

Sometimes an operating system upgrade introduces network connectivity problems. The same network adapter that worked correctly on an earlier or different operating system, such as Microsoft Windows 98 or Microsoft Windows 95, may cause problems after an upgrade. Sometimes a server relocation introduces a connectivity problem.

The most common causes for connectivity problems are:

• Network adapters and switch ports have mismatching duplex levels or transfer speed settings.
• Network adapters or switches with transmission rates of 10/100 megabits per second (Mbps) do not switch over correctly. Some autosense settings may not correctly detect the speed of some network adapters.
• The network adapter is incompatible with the motherboard or other hardware or software components and drivers.

Typical error messages include the following: The client computer's system event logs may show any one of the following entries: There may be multiple occurrences of the following TCPIP 4201 entry in the event logs: A single TCPIP 4201 event is typical after you restart the computer or after you disable or enable the network adapter.

Troubleshooting

To troubleshoot network connectivity problems, follow these steps:

1. Use the Ping or PathPing command-line tools to test basic connectivity. Use Ping to isolate network hardware problems and incompatible configurations. Use PathPing to detect packet loss over multiple-hop trips.
   
   To watch Ping statistics, use the ping -t command. To see statistics and continue, press CTRL+BREAK. To stop, press CTRL+C. If you detect lost packets in the statistics output, this indicates networking problems up to Open Systems Interconnection (OSI) layer 3 (IP-level connectivity).
   
   If the remote system that you ping is across a high-delay link, such as a satellite link, responses may take longer. Use the -w (wait) switch to specify a longer time-out.
   
   For more information about using these tools, see the "Test Network Connection with Ping and PathPing" section in the Microsoft Windows 2000 Server Resource Kit, or see "Testing the Network Connection with Ping and PathPing" in Chapter 19 of the Microsoft Windows XP Professional Resource Kit Documentation.
2. Check the event logs for network-card-related entries or connectivity-related entries.
   
   For additional information, click the following article number to view the article in the Microsoft Knowledge Base:

   308427 HOW TO: View and manage event logs in Event Viewer in Windows XP

3. Check whether the network adapter is on the Microsoft Hardware Compatibility List (HCL). To see the HCL, visit the following Microsoft Web site:

   Hardware Driver Quality

4. Check other computers that use the same default gateway that are plugged into the same hub or switch. If these computers do not experience network connectivity problems, the problem may be a faulty network adapter on one computer.
   
   If this is the case, update the network adapter driver to the latest version.
5. Contact the vendor of each motherboard and update the BIOS of the boards. Some network adapters and motherboards or BIOS versions are incompatible. Obtain the latest version from the vendor's Web site or contact your hardware vendor.
6. Check the network adapter and uplink hardware (hub or switch) for common settings. Make sure that all complementing network resources (network adapter, hub, and switch) are set to the same speed and duplex level. If the media type is set to autosense, autosensing, or autodetect, or "Auto Select," make sure that all components are autosensing correctly.
   
   On some switches, a duplex setting of Auto may cause it to use half-duplex. You may have to force it to use full-duplex.
   
   Reset the switch, restart the client, and test the connectivity.
   
   Put the client and the server on a passive hub. If communication resumes, the problem may be caused by an incorrect network switch configuration.
   
   For more information about how to configure the devices, contact your hardware vendor.
7. Manually set the network adapter of the computer that has connectivity problems to half-duplex and a lower speed.
   
   Connect the system to a switch that is configured to half-duplex and 10-Mbps, or use a 10-Mbps hub, to see if connection can be established at a lower transmission speed.
   
   To increase performance, increase the speed settings manually to 100 Mbps, and then restart the computers. Test for network connectivity loss, increase the setting to full-duplex, and then restart the computers. If network loss occurs, reduce the duplex setting and the speed to the previous settings.
8. Swap the network cable between the failing system and the hub or switch.
9. Replace the network adapter with a network adapter that has been tested and proven reliable. To do this, follow these steps:
   1. Remove the network adapter diagnostics program.
   2. Remove the network adapter in Network properties.
   3. Install the new network adapter.
10. Run Network Monitor at the same time on both ends of the network connection. After you filter the traces on the addresses of the two systems, compare both traces to see if you can see the same traffic.
    
    Use TCP Retransmit, the Network Monitor Experts tool, to detect TCP retransmissions. To do this, follow these steps:
    1. Start Network Monitor.
    2. On the Tools menu, click Experts, and then click TCP Retransmit in the left pane.
    3. Click Add to Runlist.
    4. Click Run Experts.

    If frames are missing in one of the traces, check all intermediate cabling, hubs, switches, and routers for hardware or configuration errors.
    
    In Network Monitor, view the Capture Statistics summary frame. This frame is the last frame of the trace. If it contains a value other than 0 in the following statistic counters, the connectivity problem may be caused by a hardware or configuration problem:

    STATS: MAC CRC Errors = 0
    STATS: MAC Frames Dropped due to HardWare Errors = 0

Network switches and server network adapters have to have the duplex settings matched for communication to function correctly. Both must be set to full-duplex or half-duplex. They cannot be mismatched.

The computers on a local area network (LAN) typically share a common full-duplex network medium. This configuration permits two computers to transmit data at the same time.

Connectivity problems may occur if either of the following conditions is true:

• The computer was moved to a new Ethernet switch port that automatically senses network speed. However, the computer's network adapter is configured to force full-duplex communication with a static network transfer speed setting (10 Mbps, 100 Mbps, or 1 gigabit per second [Gbps]).
• Both the Ethernet switch port and the computer's network adapter are configured to force 100-Mbps or 1-Gbps full-duplex communication. However, the Ethernet switch or the network adapter may not be able to communicate at that rate or may not be able to use full-duplex transmissions.

You can improve network performance in an Ethernet LAN environment by using full-duplex hardware. This configuration permits two-way communication between networked devices. Without full-duplex hardware, information is sent one way and then sent the other way. Packets frequently collide on the network in a half-duplex hardware configuration, and every time a collision occurs, the packets that collided must be resent. This creates even more traffic that can decrease network performance.

With full-duplex, transmit and receive paths are separate. Therefore, you can transmit and receive at the same time, and collisions are prevented. Because of the increased throughput and lack of collisions, full-duplex is more susceptible to bad cable terminations or to cable attenuation that exceeds recommended limits. This can generate data retransmissions that become sufficient to degrade performance.