On a network, link is established when two devices are able to communicate through layer 1 and layer 2 of the OSI Layer model. When link is not established, the two devices involved in the connection are not able to communicate with one another.
There are numerous reasons why two devices would not be able to establish link. These include one side not being enabled, autoengotiation mismatch, speed (1 GigE v 10 GigE) mismatch, the receive and transmit of a fiber cable needing to be switched, etc.
Networking devices such as switches and routers will have a “show interface” command which gives you some information on what is going on with a specific port. You can usually see how many bits are traveling to and from the port and if there is link on the port.
On some devices, the information will also include a “local fault” or “remote fault” message. When I first saw these messages, I was confused as to what they meant. A search of the Internet was not very helpful either. Most information about local and remote faults led me to very technical explanations of the PHY layer. There was a ton of acronyms and there wasn’t a single simple explanation of what these errors usually mean.
What Does Local Fault Mean
Local fault means that there is some issue on the local port. The port on the device that you are looking at has a problem. On a fiber connection, most often, this problem is simply that this port is not receiving any light from the other end of the connection.
No light coming from the other end of the connection can mean the other end is not plugged in or is not enabled or that the tx/rx strands need to be switched. It might also mean that the cross connect cable connecting the two devices is faulty. Whatever the reason, a local fault is usually caused by a lack of light being received.
Another cause of a local fault would be a signal mismatch. If the local port is unable to understand the incoming bits due to signals being mismatched (such as one port sending bits too rapidly) then that port displays a local fault error.
A local fault is caused by a problem on the receive path of the connection. Anytime a port cannot detect and understand a normal state on its receive path it will display a local fault error.
What Does Remote Fault Mean
When a port has a local fault error it sends a message to the remote side of the connection telling the other side that there is an error. The other side of the connection gets this error and displays it as a remote fault error.
So a remote fault error means that the other side of the connection had a local fault error. The other end of the connection is unable to clearly receive bits.
A remote fault on a device means that you should check to make sure your device is properly transmitting data.
When a remote fault is detected, the local device usually stops attempting to transmit further data. Because of this, remote fault detection is sometimes disables. There are times when you will want a device to continue to send data, even through errors, rather than cease transmission.
Not all devices are capable of local and remote fault detection. Those that are should allow you to enable and disable these features.
