Guide to Ethernet Troubleshooting

1. Are all devices powered?Power any unpowered devices
2. Are the link and activity lights lit?The Link LED indicates if there is a physical link between two Ethernet devices. The Activity LED indicates if Ethernet packets are moving across that link. If neither is lit, check RJ45 cable connections and cabling. If only one is lit, you may have a cable break (usually bad connection in the connector).
3. Are you using the correct cable?There are two kinds of Ethernet cables, Straight thru and Crossover. Straight are usually blue and Crossover are usually red. Always use a blue cable between a device and a switch or hub.
4. Have you misconnected your Fiber Optic Cable?Fiber Optic cables have a send (Tx) and a receive (Rx). The send of each device must be connected to the receive of the other.
5. How old is your Fiber cabling? As Fiber Optic cable ages, its transmission properties decline. Is it time to change your Fiber Optic Cabling?
7. Have you documented the length of your cable runs? Are you exceeding cable run standards?Standard Ethernet is good up to 100 meters.
8. Are your devices expecting the same Fiber Optic cable type? There are two main types of Fiber Optic Cable; Single Mode and Dual Mode. Single Mode cable is a single strand of glass fiber. It is the highest speed and highest cost. Multi mode cable uses multiple communication paths. It has high bandwidth over shorter distances than Single mode. Always make sure to connect Single Mode devices to Single Mode devices and Multi Mode to Multi Mode devices.
9. Is the cache in your switch refreshed? When you move a device from one port on a switch or a router to another port on the same switch or router, it can take a few minutes for the software to recognize that the device has moved and start routing packets to the new port. Power Cycling the switch or the router is a good way to force the update.
10. Are you having Auto Negotiation Problems?When you move a device from one port on a switch or a router to another port on the same switch or router, it can take a few minutes for the software to recognize that the device has moved and start routing packets to the new port. Power Cycling the switch or the router is a good way to force the update.
11. Is this a noisy environment? Sometimes switches and routers are unable to negotiate the correct link speed with a device. The correct speed is never established. You may be able to disable this auto negotiation process.
12. Are your devices getting valid TCP/IP addresses from the DHCP server?A DHCP server provides a device with its TCP/IP addresses. If the DHCP server fails or is unavailable, your devices can not get a valid TCP/IP address and can not join the network properly. Check one or two of your nodes that support some a web server or front panel that can display the TCP/IP address for valid TCP/IP addresses. Power cycle those devices and check them again to see if they received a valid TCP/IP address from the DHCP server after the power cycle.
13. Can your devices be connected to the other devices on the network?When a router or switch in a complex architecture fails, the devices that use that switch to transmit packets will lose that connectivity. Join the network with your laptop and issue a PING command to each device in your network to test if all your switches and routers are working properly.
14. Is your network architecture setup properly?Devices can only talk to other devices on the same subnet. That means that the xxx.yyy.zzz portion of the TCP/IP addresses must be identical. To access devices in a different subnet, you must have a router that can take messages off one subnet and resend them on a new subnet. Check and Document your entire network architecture.
15. Are your switches and routers configured properly?Switches and routers can be very complex with lots of features to manage. Have any changes made to your switch or router setup? Is a port disabled?
16. How are your cables physically routed?Have you routed your cables near high power lines? Poor bandwith is sometimes due to noisy effects from nearby cabling.
17. Is your problem a connection problem or a protocol problem?Connection problems are issues with cabling, switches, network architecture and the like. Connection problems prevent the movement of packets from one Ethernet device to another Ethernet device. Protocol problems are the result of two devices sending and receiving packets correctly but failing to understand the data in those packets. Here are some tools that can assist you in solving those kinds of problems.