Layer and Interface Status

Physical Troubleshooting

Using the LEDs to Troubleshoot

If you have physical access to the switch, it can save time to look at the port LEDs which give you the link status or can indicate an error condition (if red or orange).

Ensure that both sides have a link. A single broken wire or one shutdown port can cause the problem where one side has a link light, but the other side does not.

A link light does not guarantee that the cable is fully functional. The cable can have encountered physical stress that causes it to be functional at a marginal level. Normally you can identify this situation if the port has many packet errors, or the port constantly flaps (loses and regains link).

Check the Cable and Both Sides of the Connection

If the link light for the port does not come on, you can consider these possibilities:

Possible Cause Corrective Action
No cable connected Connect cable from switch to a known good device.
Wrong Port Make sure that both ends of the cable are plugged into the correct ports.
Device has no power Ensure that both devices have power.
Wrong cable type Verify the cable selection. 
Bad cable Swap suspect cable with known good cable. Look for broken or missing pins on connectors.
Loose connections Check for loose connections. Sometimes a cable appears to be seated in the jack, but is not. Unplug the cable and reinsert it.
Patch Panels Eliminate faulty patch panel connections. Bypass the patch panel if possible to rule it out.
Media Convertors Eliminate faulty media convertors: fiber-to-copper, etc. Bypass the media convertor if possible to rule it out.
Bad or wrong Gigabit Interface Convertor (GBIC) Swap suspect GBIC with known good GBIC. Verify Hw and Sw support for this type of GBIC. 
Bad Port or Module Port or Interface or Module not enabled Move the cable to a known good port to troubleshoot a suspect port or module. Use the show port command for CatOS or the show interface command for Cisco IOS to look for errdisable, disable or shutdown status. The show module command can indicate faulty, which can indicate a hardware problem. 

Ethernet Copper and Fiber Cables

Make sure you have the correct cable for the type of connection you are making. Category 3 copper cable can be used for 10 Mbps unshielded twisted pair (UTP) connections, but must never be used for 10/100 or 10/100/1000Mbps UTP connections. Always use either Category 5, Category 5e, or Category 6 UTP for 10/100 or 10/100/1000Mbps connections.

Warning: Category 5e and Category 6 cables can store high levels of static electricity because of the dielectric properties of the materials used in their construction. Always ground the cables (especially in new cable runs) to a suitable and safe earth ground before you connect them to the module.

For fiber, make sure you have the correct cable for the distances involved and the type of fiber ports that are used. The two options are singlemode fiber (SMF) or multimode fiber (MMF). Make sure the ports on the devices that are connected together are both SMF, or both are MMF ports.

Note: For fiber connections, make sure the transmit lead of one port is connected to the receive lead of the other port. Connections for transmit-to-transmit and receive-to-receive do not work.

Ethernet and Fast Ethernet Maximum Transmission Distances

Transceiver Speed Cable Type Duplex Mode Maximum Distance Between Stations
10 Mbps Category 3 UTP Full and half 328 ft (100 m)
10 Mbps MMF Full and half 1.2 mi (2 km)
100 Mbps Category 5 UTP Category 5e UTP Full and half 328 ft (100 m)
100 Mbps Category 6 UTP Full and half 328 ft (100 m)
100 Mbps MMF Half 1312 ft (400 m)
    Full 1.2 mi (2 km)
100 Mbps SMF Half 1312 ft (400 m)
    Full 6.2 mi (10 km)

For more details on the different types of cables/connectors, cabling requirements, optical requirements (distance, type, patch cables, etc.), how to connect the different cables, and which cables are used by most Cisco switches and modules.

Gigabit Ethernet Troubleshooting

If you have device A connected to device B over a Gigabit link, and the link does not come up, perform this procedure.

Step-by-Step Procedure

  1. Verify device A and B use the same GBIC, short wavelength (SX), long wavelength (LX), long haul (LH), extended wavelength (ZX), or copper UTP (TX). Both devices must use the same type of GBIC to establish link. An SX GBIC needs to connect with an SX GBIC. An SX GBIC does not link with an LX GBIC. 

  2. Verify distance and cable used per GBIC as defined in this table.

    1000BASE-T and 1000BASE-X Port Cabling Specifications

    GBIC Wavelength (nm) Copper/Fiber Type Core Size1 (Microns) Modal Bandwidth (MHz/km) Cable Distance2
    WS-G5483 1000Base - T (copper)   Category 5 UTP Category 5e UTP Category 6 UTP     328 ft (100 m)
    WS-G5484 1000BASE-SX3 850 MMF 62.5 62.5 50.0 50.0 160 200 400 500 722 ft (220 m) 902 ft (275 m) 1640 ft (500 m) 1804 ft (550 m)
    WS-G5486 1000BASE-LX/LH 1310 MMF4 SMF 62.5 50.0 50.0 8.3/9/10 500 400 500 - 1804 ft (550 m) 1804 ft (550 m) 1804 ft (550 m) 6.2 miles (10 km)
    WS-G5487 1000BASE-ZX5 1550 MMF SMF6 8.3/9/10 8.3/9/10   43.5 miles (70 km)7 62.1 miles (100 km)


    1. The numbers given for multimode fiber-optic cable refer to the core diameter. For single-mode fiber-optic cable, 8.3 microns refers to the core diameter. The 9-micron and 10-micron values refer to the mode-field diameter (MFD), which is the diameter of the light-carrying portion of the fiber. This area consists of the fiber core plus a small portion of the surrounding cladding. The MFD is a function of the core diameter, the wavelength of the laser, and the refractive index difference between the core and the cladding.

    2. Distances are based on fiber loss. Multiple splices and substandard fiber-optic cable reduce cabling distances.

    3. Use with MMF only.

    4. When you use an LX/LH GBIC with 62.5-micron diameter MMF, you must install a mode-conditioning patch cord (CAB-GELX-625 or equivalent) between the GBIC and the MMF cable on both the transmit and receive ends of the link. The mode-conditioning patch cord is required for link distances less than 328 feet (100 m) or greater than 984 feet (300 m). The mode-conditioning patch cord prevents overdriving the receiver for short lengths of MMF and reduces differential mode delay for long lengths of MMF. 

    5. Use with SMF only.

    6. Dispersion-shifted single-mode fiber-optic cable.

    7. The minimum link distance for ZX GBICs is 6.2 miles (10 km) with an 8-dB attenuator installed at each end of the link. Without attenuators, the minimum link distance is 24.9 miles (40 km).

  3. If either device has multiple Gigabit ports, connect the ports to each other. This tests each device and verifies that the Gigabit interface functions correctly. For example, you have a switch that has two Gigabit ports. Wire Gigabit port one to Gigabit port two. Does the link come up? If so, the port is good. STP blocks on the port and prevents any loops (port one receive (RX) goes to port two transmit (TX), and port one TX goes to port two RX).

  4. If single connection or Step 3 fails with SC connectors, loop the port back to itself (port one RX goes to port one TX). Does the port come up? If not, contact the TAC, as this can be a faulty port.

  5. If steps 3 and 4 are successful, but a connection between device A and B cannot be established, loop ports with the cable that adjoins the two devices. Verify that there is not a faulty cable.

  6. Verify that each device supports 802.3z specification for Gigabit auto-negotiation. Gigabit Ethernet has an auto-negotiation procedure that is more extensive than the one used for 10/100 Ethernet (Gigabit auto-negotiation spec: IEEE Std 802.3z-1998). When you enable link negotiation, the system auto-negotiates flow control, duplex mode, and remote fault information. You must either enable or disable link negotiation on both ends of the link. Both ends of the link must be set to the same value or the link cannot connect. Problems have been seen when you connect to devices manufactured before the IEEE 802.3z standard was ratified. If either device does not support Gigabit auto-negotiation, disable the Gigabit auto-negotiation, and it forces the link up. It takes 300msec for the card firmware to notify the software that a 10/100/1000BASE-TX link/port is down. The 300msec default debounce timer comes from the firmware polling timer to the linecards, which occurs every 300 msec. If this link is run in 1G (1000BASE-TX) mode, Gigabit sync, which occurs every 10msec, must be able to detect the link down faster. There is a difference in the link failure detection times when you run GigabitEthenet on copper versus GigabitEthernet over Fibre. This difference in detection time is based on the IEEE standards.

    Warning: Disabling auto-negotiation hides link drops or physical layer problems. Disabling auto-negotiation is only required if end-devices such as older Gigabit NICs are used which cannot support IEEE 802.3z. Do not disable auto-negotiation between switches unless absolutely required to do so, as physical layer problems can go undetected, which results in STP loops. The alternative is to contact the vendor for software/hardware upgrade for IEEE 802.3z Gigabit auto-negotiation support.

Checking Interfaces Status

You can view summary or detailed information on the switch ports using the show interfaces status command. To see summary information on all ports on the switch, enter the
show interfaces status command with no arguments. Specify a particular module number to see information on the ports on that module only. Enter both the module number and the port number to see detailed information about the specified port.

To apply configuration commands to a particular port, you must specify the appropriate logical module.

This example shows how to display the status of all interfaces on a Catalyst 4500 series switch, including transceivers. Output of this command displays "Unapproved GBIC" for non-Cisco transceivers:

Switch#show interfaces status

Port    Name               Status       Vlan       Duplex  Speed Type

Gi1/1                      notconnect   1            auto   auto No Gbic

Gi1/2                      notconnect   1            auto   auto No Gbic

Gi5/1                      notconnect   1            auto   auto 10/100/1000-TX

Gi5/2                      notconnect   1            auto   auto 10/100/1000-TX

Gi5/3                      notconnect   1            auto   auto 10/100/1000-TX

Gi5/4                      notconnect   1            auto   auto 10/100/1000-TX

Fa6/1                      connected    1          a-full  a-100 10/100BaseTX

Fa6/2                      connected    2          a-full  a-100 10/100BaseTX

Fa6/3                      notconnect   1            auto   auto 10/100BaseTX

Fa6/4                      notconnect   1            auto   auto 10/100BaseTX


This example shows how to display the status of interfaces in error-disabled state:

Switch# show interfaces status err-disabled

Port    Name               Status         Reason

Fa9/4                      err-disabled   link-flap

informational error message when the timer expires on a cause


5d04h:%PM-SP-4-ERR_RECOVER:Attempting to recover from link-flap err-disable state on Fa9/4