No more worries about network failures: Understand Ethernet interface physical DOWN failures and solutions in one article!

1. Concept of Interface Physical Down

The Ethernet interface is a real physical interface supported by devices and provided for user operation or configuration. It is mainly used to receive and send data. It is the most basic condition for the normal operation of the device that the interface remains in the UP state.

When an interface is in DOWN state, its indicator is off and the interface cannot receive or receive packets normally. A log similar to the following is generated on the device:

 IFPDT/4/IF_STATE(l)[3]:Interface GigabitEthernet0/0/12 has turned into DOWN state.

You can run the display interface command to view the current physical status of the interface (information in the current state field in the command output).

In the example in the preceding figure, the current physical status of interface GE0/0/12 is Administratively DOWN. That is, the shutdown command exists in the device interface. It can be concluded that a configuration error causes the interface to go down. You need to run the undo shutdown command in the corresponding interface view.

2. Causes and Solutions for Physical Down of Interfaces

The main reasons for the physical down status of an interface include hardware and software failures:

• Hardware failure: Hardware failures on the local or remote devices include single boards, interfaces, optical modules, optical fibers, network cables, etc.
• Software failure: mainly includes inconsistent configurations at both ends of the link, including port negotiation mode, rate, duplex and other configurations.

As shown in the figure below, in order to more accurately analyze the cause of the physical DOWN of the interface, this article mainly takes the possible introduction point of the fault as a clue, and specifically follows the five steps of man->machine->material->method->environment as the troubleshooting sequence to gradually find the root cause of the problem and eliminate the fault.

The interface is physically down.

1. Human factors cause the interface to be physically down

(1) Description of the fault cause

When the port changes from the normal UP state to the physical DOWN state, you need to first check whether there is any human operation in the recent period, including incorrect plugging and unplugging of network cables, accidental contact with devices resulting in loose connections, and incorrect command line operations on the device.

(2) Troubleshooting steps

In any view on the two interconnected devices, run the displayinterface interface-type interface-number command to check the current interface status and interface statistics. As shown in Figure 3, focus on the red fields in the following output:

• If the current state field displays Administratively down, it indicates that the interface has been shut down manually. In this case, run the undo shutdown command on the interface.
• If the Current State field is DOWN, check whether the rate, duplex mode, and auto-negotiation mode are consistent at both ends of the link.
• If the Negotiation field is "ENABLE", the interface is in auto-negotiation mode. The rate and duplex are the result of negotiation between the two parties. If the rate and duplex negotiation at both ends of the link are inconsistent, you can run the restart command to restart the interface to renegotiate. If the negotiation is still unsuccessful, run the undonegotiation auto command to make it work in non-auto-negotiation mode. For the rate and duplex configuration, refer to the next situation.
• If the Negotiation field is DISABLE, the interface is in non-auto-negotiation mode. To configure the dual-rate mode, you can execute speed[10|100|1000] and duplex{full half} in the interface view to ensure that the parameters at both ends of the link are consistent.
• If the current state field is ERROR DOWN (down-cause), it means that the interface is shut down due to an error event. You need to troubleshoot the fault according to the specific field information of down-cause. Common down-cause field values ​​are shown in the following figure.

(3) "Failure recovery method

Batch interfaces are physically DOWN:

• If multiple interfaces are physically DOWN, and these interfaces belong to the same board and are connected to multiple peer devices without abnormalities, the board may be abnormal. Try to execute the reset slot slot-id command to reset the board. If the fault is still not resolved, you need to replace the board.
• If multiple interfaces are physically DOWN, and these interfaces are interfaces that connect the local device to the same device on the other end, the possible reason is that the other end device is abnormal. You need to log in to the other end device to check whether the device board is faulty, and reset or replace it in time. If the other end device is in the state of active-standby switching, sleep, restart, etc., it can be considered a normal phenomenon. After the state of the other end device stabilizes, the state of the local device interface can automatically return to the UP state. In this case, the interface will appear UP/DOWN frequently, generating a large number of UP/DOWN logs as shown below.

A single interface is physically down: If only a single interface is physically down, check whether there are hardware faults such as dented metal shrapnel on the interfaces at both ends of the link. If the interface is faulty, try using another idle interface.

2. Material--connection medium failure

(1) Description of the fault cause

The physical interfaces of the switch include electrical interfaces and optical interfaces. The transmission media involved are mainly RJ45 network cables, optical modules, and optical fibers. When the transmission media is aged or damaged, or when the optical module transmits or transmits light abnormally, the interface will be physically down.

(2) Troubleshooting steps

The electrical port connection medium is faulty:

• Check whether the connection is correct: ensure that the interfaces at both ends of the connection use the same network cable. If it passes through a jumper rack, check whether the jumper is correct, check whether the interface is connected correctly, and check whether there is poor contact and external damage to the network cable.
• Check whether the specifications and length of the network cable meet the standards: According to the relevant Ethernet standards, the length of the network cable cannot exceed 100m. For GE electrical ports, if you use a network cable lower than Category 5, the GE electrical port will not be able to UP at a rate of 1000M. For all electrical port connections, it is recommended to use a network cable with specifications above Category 5.
• Check whether the internal line pair status of the network cable is normal: if some of the 4 pairs of lines in the network cable are damaged, the electrical port cannot be guaranteed to be up normally. To check the quality of the network cable, you can use a cable tester or execute the virtual-cable-test command in the interface view to check whether the line pair connection is normal.

As shown in the figure above, if the Pair state in the echo is OK, it is normal; if it is other states (such as Open or short), it is recommended that you change another network cable for testing.

The optical port connection medium is faulty:

• Whether the optical module is a Huawei-certified optical module: Huawei adds the Huawei logo and corresponding identification on the optical module label of the switch product, as shown in Figure 7. The manufacturer information in the product electronic label and optical module information is updated to "HUAWVEI". If an optical module that is not Huawei-certified is inserted into a switch device, an alarm message will appear. The alarm format is as follows:

 ENTITYTRAP/3/OPTICALINVALID:OID oid] Optical Module is invalid.(Index=INTEGER), EntityPhysicalIndex=INTEGER), PhysicalName= "OCTET", EntityTrapFaultID=[INTEGERJ)

• Check whether the optical module is in normal status: Run the display transceiverinterface command to check whether the optical modules on both ends are working normally. Check the received optical power, transmitted optical power, and current value to ensure that all indicators are within the normal range.

(3) Fault recovery method

Electrical port connection medium failure: If the electrical port network cable fails, it is recommended to directly replace the network cable.

The optical port connection medium is faulty:

• If the optical module you are using is not a Huawei-certified optical module, please replace it with a Huawei-certified optical module. The reliability of non-certified optical modules cannot be guaranteed, which may cause service instability. Huawei will not be responsible for problems caused by non-Huawei-certified optical modules, and in principle will not provide solutions.
• If you run the display transceiverinterface command to view the working status parameters of the optical module, determine whether the parameters are outside the normal range according to the standard conclusions listed in Table 1.

• If the transmit optical power is too low or too high: Please replace the optical module
• If the received optical power is too low or too high: Use an optical power meter to test the optical power reduction of each part of the link and repair the abnormal parts.
• If the problem still cannot be solved, please replace the optical module or optical fiber to ensure that the transmission distance of the optical module and the type of optical fiber meet the networking requirements.

3. Method - Wrong connection medium operation method

(1) Troubleshooting steps

When performing media connection operations between devices, you need to consider whether the optical module matches the optical port, whether the optical module and the optical fiber model are dead-matched, whether the optical module and the optical fiber connection operation are correct, etc. When the interface is physically DOWN, you can check from the above reasons.

(2) Troubleshooting steps

• Execute the display device command in any view to view the device model and component information. For frame-type devices, you can view the corresponding board name.

• Log in to the hardware query tool to check whether the optical module type currently used matches the corresponding device interface and whether the optical module matches the optical fiber model;
• Run the display transceiver interface command in any view on both ends of the link to check whether the optical module parameters at both ends are consistent.

• Check whether the optical fiber connection method is correct. The optical module generally includes a transmitting end (TX) and a receiving end (RX). When the optical interface is connected, two optical fibers are required to connect the TX end of one optical module with the RX end of the other end, and the RX end of one optical module with the TX end of the other end;

• Determine whether the interface is a combo interface. If it is a combo interface, check whether the interface is used correctly, that is, whether the current working mode of the interface is consistent with your current interface connection.

As shown in the figure below, the Combo interface GE1/0/1 corresponds to two interfaces on the device panel: a Combo optical interface and a Combo electrical interface. The Combo optical interface and the Combo electrical interface cannot work at the same time. When the user needs to configure the attributes of the Combo optical interface or the Combo electrical interface, they only need to enter the same interface view GE1/0/1 to configure it.

Run the display interface command in any view to check whether the interface is a combo interface and whether the current working mode is normal.

picture

(3) Fault recovery method

Use the hardware query tool to ensure that the optical module currently in use matches the optical port and the optical module matches the optical fiber. If they do not match, replace the corresponding parts.

If the current interface is a combo interface, the principles for selecting the combo interface working mode are as follows:

• When the Combo electrical port is not connected to a network cable and an optical module is plugged into the Combo optical port, the Combo port selects the optical port mode.
• The Combo electrical port is connected to a network cable and is in UP state. Even if an optical module is plugged into the Combo optical port, the Combo port is still in electrical port mode. However, after the device is restarted, the Combo port will change to optical port mode.
• The Combo electrical port is connected to a network cable and is in DOWN state. When an optical module is inserted into the Combo optical port, the Combo port will select the optical port mode.
• When the combo interface working mode is set to automatic selection mode (execute comboport auto in the interface view, the default device combo interface working mode is auto), the system will detect whether there is an optical module inserted in the combo optical port and select the mode:

In summary, when the combo interface works in automatic selection mode, as long as the combo optical port has an optical module installed, the combo interface will select the optical port mode after the device restarts.

When the working mode of a combo interface is set to forced mode, you need to configure the following based on the type of the interface connecting the local end and the peer device:

• If the local Combo electrical port is connected to the peer electrical port, you need to forcibly set the working mode of the Combo interface to copper. That is, execute the combo-port copper command in the connection view.
• If the local Combo optical port is connected to the remote optical port, you need to set the working mode of the Combo interface to fber. That is, execute combo-port fiber.

In summary, after determining the correct transmission medium (optical fiber or network cable), you can determine the correct working mode of the Combo port. You can directly use the combo-port {copper|fiber] command to force the configuration of the interface working mode to be consistent with the connection medium.

4. Environment--Failures caused by environmental factors

(1) Cause description

The use scenarios of switches are complex and diverse, and devices can be at any level in the network. When the switch and its connection media are abnormal due to environmental factors, the interface status may also be physically down.

(2) Troubleshooting steps

• Check the temperature and humidity of the equipment room: The operating environment temperature of the S switch is 0C~45C, and the humidity should be between 5%RH~95%RH, without condensation.
• Check the cable layout: Power cables and service cables should be laid separately. Strong and weak power cables should not be placed in parallel to prevent interference. Check whether the network cables are damaged or aged due to high temperature or rack wear.

(3) Recovery method

• If the ambient temperature of the computer room cannot meet the requirements for a long time, you should consider repairing or replacing the air conditioning system of the computer room. If the relative humidity of the computer room is too high, you should consider installing dehumidification equipment in the computer room;
• If the relative humidity in the computer room is too low, you should consider installing a humidifier in the computer room. If strong and weak power cables are parallel, please redeploy the cable directions and separate the strong and weak power cables. If the network cable is damaged and aged due to environmental factors, please replace the network cable.