1. DEAD DROP
Symptom: A PC, phone, access point, or printer is connected to the wall jack and the connection is not activated. The switch port does not show a link light, nor does the network adapter.
Cause: Usually happens when a connection is not patched through to the wall jack. When offices or meeting rooms are moved around, sometimes the network jacks are not tested for the new users, or drops may not be properly documented. Or the switch port may be administratively disabled.
Resolution: Check to be sure the switch port is activated and the connection has been patched. When any devices have been physically moved, be sure to test the new connections to ensure they are working properly. In the case of an IP phone, it may also be a power supply issue.
2. NO IP ADDRESS
Symptom: The network appears down or inoperable. The OS may alert that an address was not received from the DHCP server. After checking the network adapter status, no address may be configured.
Cause: The DHCP server may be out of addresses, the service may be down, the end device may be configured to use a static address instead of a DHCP address, or the DHCP request from the end device never made it to the server in the first place. This may be the case especially if a new device is configured for a VLAN that is not set up to forward DHCP requests to the DHCP server.
Resolution: Is this problem restricted to one user or many? If only one, check that the NIC settings are configured for DHCP. Next, check the switch to see which VLAN the port is configured. Check that other devices on this VLAN can get addresses. If they cannot, the problem may be that the router is not forwarding DHCP requests to the DHCP server. If many devices across several subnets are having this problem, the problem is likely the server itself.
3. NO CONNECTION TO APPLICATION SERVER
Symptom: The application the user is trying to open may alert that it cannot connect to the application server.
Cause: The key question is whether this problem happens constantly or only sporadically. If the user has a proper IP address for the connection, there may be a routing issue on the network. If connectivity is lost sporadically, this can be caused by a busy server.
Resolution: If routing is not the issue (use ping test), check the server load and resources. Is the server busy running another task such as a backup? If not, check the network load between client and server, focusing on WAN connections, if there are any. Look for Ethernet errors on all switches and routers that cause packet loss between client and server.
4. SLOW APPLICATION PERFORMANCE
Symptom: The application appears sluggish. It may freeze on a certain screen or halt while accessing data. Often, the network is blamed for these issues.
Cause: Exonerating the network in application performance problems can assist server maintenance personnel to take out the guesswork and isolate the issue to the right place. Many issues can cause an application to slow down. Among the most common causes are server backups occurring during production hours, slow response from database servers, and packet loss on the network. From a network technician’s point of view, the most important thing to determine is whether the problem is caused by the server or by the network. To determine this, a capture of application traffic can be collected from a client machine. Look for any retransmissions between the client and server. If retransmissions exist, there is packet loss on the network, which severely affects application performance. If there are no retransmissions and connectivity between client and server is established, the problem is likely in the server and can be monitored from that perspective.
Resolution: Although packet analyzers can be very difficult to use when tracking down a problem, they often are equipped with simple counters displaying TCP retransmissions. Use this counter to assist in determining if there is packet loss on the network between client and server. Look for Ethernet errors (FCS errors, alignment errors, or late collisions) on any switches and routers between client and server that could be causing this packet loss. If there are no errors, watch for packet loss on the WAN because of excessive utilization across the link.
5. WIRELESS CLIENT CAN’T CONNECT
Symptom: The client can detect the wireless access point, but it cannot connect to the wireless network.
Cause: Security credentials, wireless channel interference, and dead spots can cause this problem.
Resolution: Using a wireless monitoring tool, measure the signal strength from the affected area and, if possible, conduct a site survey in the area to look for rogue or unknown access points (APs). These may be configured for overlapping wireless channels and can affect known good users. Check for noise in the signal from surrounding access points or from noise-inducing devices such as microwaves and cordless phones. Monitor the client as it attempts to connect to the access point and watch for which step fails — association, authentication, or authorization.