Physical layer issues are issues that pertain to the physical interface between devices. In most cases, this term is used to refer to problems with the wires connecting devices, or problems with the devices connecting to the network.
These can be very tricky problems to diagnose and fix. When there is a problem with the physical layer of the network, it can cause a lot of confusion as to where the problem originates.
Diagnosing these problems is part art, part science. The best technicians have a sharp eye for detail and can pick up on small things that seem irrelevant, but make a difference.
General solutions for physical layer issues usually include restarting both ends of the connection and re-establishing the connection. Re-booting your device may sound like a simple solution, but it can be tough if your device does not have an internal reset button.
Verify the link is up
Next, you should verify that the link is up. You can do this by pinging the remote end of the link or using show commands to check the status of the interface.
The show interface command will display a number of things about the interface, including the physical layer state of up or down. It will also display any errors that may be occurring on the link.
A common problem beginners have is verifying that the show interface command shows an OK condition for the Physical Layer state. Many think that if they can ping their destination then the link is up, but this is not always true!
If you are having trouble with your Physical Layer, try checking out some other tips on this website to further troubleshoot your issue(s)>.
Verify the cabling is correct
The first thing you should do is make sure the cabling is in correctly. The way to do this is to check the physical connection of the cables to the ports.
You can also verify that the link is working by using the show interface command to check if there are any errors and that packets are being received and transmitted.
The show interfaces command uses IPC (inter-processor communication) to communicate information between the CPU and I/O devices within a device. This allows all devices in a device family to use the same command, making it easier to understand for users and technicians.
By using this, you can also verify that all modes of the interface are working, such as switching, bridging, and routing. You can also verify if all protocols are functioning properly.
Replace the cable if needed
If there is physical damage to the cable, then you should replace the cable. The reason for this is because there may be damage to the wires inside of the cable, and trying to fix it may just cause more problems.
Corrosion or cuts in the wire coatings can also cause issues with the data being transmitted through the wire. The electronics in the router or computer cannot interpret what is happening with the wire anymore, so it needs to be replaced.
There are many sources for cheap cables online, but purchasing quality cables at a store may be a better investment in the long run. Buying from a store that specializes in cables can also help determine if your cable is good or not.
Sending your wires back to where they came from may be helpful as well.
Run the show interfaces gi0/1 command and verify the following information
The show interfaces command is a great tool for verifying and troubleshooting physical layer issues. You can verify the configuration of the interface, check for errors, and check the throughput of the link.
The following example shows a test run of the show interfaces gi0/1 command. Check out this example to see if your results match!
Verification of the configuration on interface Gi0/1 indicates that the configuration is not in error. The Physical layer protocol is PPP and the IP address information is correct. The Link type is Ethernet and the Broadcast type is disabled.
The next part of this example shows that there are no errors on interface Gi0/1. This section also shows that there is a 64 kbps transmission rate, which matches what was configured for the bandwidth on this link.
The status should be up/up
A common problem that network technicians encounter is a physically down network connection. This can be due to a number of things, including disconnected or incorrect cables, incorrect IP settings, and wrong firewall rules.
Once you have established that the problem is not on your side of the connection, you should check the interface status on the device at the other end of the connection.
The status of an interface can be checked using the show interface command. You can also verify whether or not the ports are enabled using this command.
If the port is enabled but there is no connectivity, then you may need to troubleshoot your network addressing and configuration. This could be due to wrong address formats, insufficient security measures, or no security at all.
Once you have verified that your networking equipment is configured correctly, then you should check to see if there are any issues with power.
Link status should be ok
The next step is to check the link status of the interface. A common way to do this is with the show interface command. This shows you many things about the interface, one of which is the link status.
A link status of down indicates that no physical connection exists and no Layer 3 configuration is present. A link status of dormant indicates that a physical connection exists, but no Layer 3 configuration is present.
These two situations can be differentiated by issuing a ping command. If a ping fails, the interface is considered down. If a ping succeeds, then it is not configured for IP address so it is the Layer 3 functionality that needs to be fixed.
There are several other link statuses that can be returned such as notconnect, initialize-error, and failedinitialize. These indicate different problems with the physical connection or initialization.
CDP status should be ok
The Cisco Discovery Protocol (CDP) is a proprietary protocol developed by Cisco to help users manage their networks. It is a feature most people do not use, but it can be extremely helpful in troubleshooting physical layer issues.
All managed switches and some managed routers have the ability to send CDP messages to attached devices. By default, CDP is turned on for all interfaces on Cisco devices.
CDP packets contain information about the device itself, as well as information about the adjacent ports. This information can be very useful when trying to find what is wrong with a particular link.
If there is no CDP status transmitted on the interface, then you should check if there is any other physical layer issues by having other devices connect to the interface and checking for connectivity.
Speed should be 1000 Mbps
The first thing you should check is the speed of the interface. You can do this by issuing the show interfaces gi0/1 command and checking the bandwidth and duplex settings.
The show interfaces command also shows you the current configuration of the interface, so if speed is not an issue, then you know to check other things.
Speed issues can be caused by either end of the connection. If your router interface is set to 1000 Mbps but your PC device is set to 100 Mbps, then there will be a problem.
Duplication issues can also cause problems. If your interface is set to auto-duplex, then it will work if the connected device is also set to auto-duplex. If it is set to full-duplex but the connected device is set to half-duplex, then there will be a problem.
Checking these settings before proceeding with other troubleshooting steps can help narrow down the problem.
