Faults of Physical Layer

I. Fault Description

The following faults occur at the physical layer:

z The state of the physical port is always DOWN.

Execute the command display port to confirm it.

z The state of the physical port is UP. A large amount of packets are lost when the

peer port is pinged.

Execute the command display port to confirm it.

II. Fault Analysis

When a port is DOWN, there may be two kinds of states:

z The port is shutdown.

The system prompts that port (such as serial number) is administratively down, and line protocol is down.

z The port is not activated or the state of the physical layer is not changed to up.

The system prompts that port (such as serial number) is down, and line protocol is down.

Loss of packets is caused by the fault of line clocks. This fault generally occurs on the private line of a WAN port.

III. Troubleshooting

Table A-1 lists causes and troubleshooting procedures of faults.

Table A-1 Cause and troubleshooting procedure

Cause Troubleshooting

For the first case, you can active the port by executing the command undo

shutdown.

Port is shut down. For the second case, check whether the states of the DTR, DSR, RTS, CTS, and DCD signals at the lower layer are all up by executing the command

display port. If not, it indicates that the physical link between the DTE and DCE

is faulty. Check the connection of cables. Packet loss is

Note:

Meanings of the above five signals are as follows:

z DTR: Signal of data terminal equipment (DTE) is ready (input signal). z DSR: Signal of data connection equipment (DCE) is ready (output signal). z RTS: Request transmission signal (input signal).

z CTS: Clear transmission signal (output signal). z DCD: Data carrier detection signal (output signal).

By default, the Quidway router detects the above five signals. When the asynchronous port transmits data, the router automatically detects the CTS signal. If there is a CTS signal, the data is transmitted normally. If there is no CTS signal, the data transmission is stopped. You can deactivate this function by setting the relevant parameters of the serial port executing the following command:

[Quidway-Serial0]#flowcontrol normal

If the flow control mode of the asynchronous serial port is set to normal, the router does not detect the CTS signal and the data is transmitted directly at the port. In this case, if transmission error occurs, the system automatically transmits the data again.

DTR, DSR, and DCD signals have the level detection function of the serial port, that is, the system detects whether the serial port is connected to external cables and detects the DCD signal. The system considers that the serial port is in UP status only when this signal is valid. Otherwise, it is in DOWN status. You can deactivate this function by executing the following command:

[Quidway-Serial0]#undo detect dsr-dtr

Caution:

z For the WAN port, the router is equipped with a variety of interface cables such as V.24 and V.35, and

the DTE and DCE. Check whether the WAN port of the router works in the synchronous or asynchronous mode. If it is in asynchronous mode, check the setting of the baud rate. If it is in synchronous DCE mode, the clock is generated by the router. Then check the clock rate and the clock mode.

z For the Ethernet port, check whether the connection of the Ethernet is proper. If the HUB or the LAN

Switch is used to connect the Ethernet, check whether the Ethernet ports of the tester and the router displayed by the indicators on the HUB or the LAN Switch are normal. Two operational modes including full-duplex and half-duplex are defined in 10Base-T Standard. When a shared HUB is used, the operational mode must be in the half-duplex mode. When a switched Switch is used, the operation can be in the full-duplex mode if the full-duplex mode is set on the switch.

z The DTE and DCE mentioned here differ from those of the physical interface. The DTE here refers to

user equipment such as router, and the DCE refers to equipment such as switch. Routers can also be used as DCE.

IV. Solution to Common Problems

Table A-2 lists solutions to some common problems.

Table A-2 Solutions to common problems

Description Analysis Solution

After two 2501 are connected, a large amount of packets are lost when the two opposite ends ping each other.

This may be because the clock of lines is non-synchronous, thus causing routers on both ends of a line not to transmit and receive signals synchronously. After the transmit-clock is inverted on the interface, the clock becomes normal.

If the fault still exists after the inversion, and the configuration is correct, you need to check connection of cables.

Invert the transmit-clock on the serial ports connected to the private line at both ends by executing the command

Invert transmit-clock.

When checking the status of serial ports of a router, you find the physical port is DOWN and statuses of the following signals are as follows:

DCD=UP DTR=DOWN DSR=UP RTS=DOWN CTS=UP

Both DTR and RTS signals are sent from DTE (data terminal). It may be the poor connection of DTE equipment that causes this problem. Check whether the DTE equipment is correctly connected and whether cables and lines are faulty.

Distinguish the cable on the configuration port from the cable on the standby port of the Quidway router.

The DB9 (25) connector of the cable on the configuration port is DCE connector, and that of the cable on the standby port is DTE connector. During the configuration of the router, the router is regarded as DCE, and it is regarded as DTE when connected to the MODEM through the AUX port.