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| 3.1 |  | Characteristics of Physical Layer Failure Problems |
| 3.1.3 |  | Noncritical characteristics – equipment indicators |

The Light Emitting Diodes (LEDs) on a device can give feedback for diagnosing the operational status of the device. When there is a physical problem with equipment, the LEDs of the failing device are usually off, flashing, or a different color than usual.

Most LED link lights are now software controlled, so they are no longer reliable as a sole indicator of connectivity. If the link light is illuminated, it may or may not mean that a valid link is present. If the port is faulty, it may be possible to disconnect the cable and still have the link light illuminated. If the link light is off however, then it is still a fairly good indication that no link is present. For devices using multispeed interfaces such as 10/100 and 10/100/1000 NICs, it is not uncommon to see link lights at one end of a failing link, but not at the other.

If a 10BASE-T device is connected to a switch that is ‘hard-set’ or only capable of 100BASE-TX, the link LED on the 10BASE-T device will often show that the link is active, but the network card will fail to communicate. The 100BASE-TX switch will not show link, and the 100BASE-TX Ethernet segment is unaffected.

If a 100BASE-TX only station is connected to a shared media 10BASE-T hub, then it will not show a link. The workstation will cause somewhere between 33 percent and 100 percent collisions on the 10BASE-T collision domain. The 10BASE-T hub will show the link, and if it has status LEDs it will usually also show constant utilization and/or collisions with those status LEDs. To troubleshoot mismatched speeds, a more thorough examination than just looking at the link state LED is needed.

If a cable fault is the cause of link failure, an auto-negotiating 10/100 device may successfully negotiate a link to 100BASE-TX, but it may not be able to subsequently establish and maintain signaling synchronization. This results in the link light blinking on and off as the Ethernet chipset repeatedly negotiates a connection speed, then fails to link. If the cable fault is not severe, then the link may come up and operate very poorly for a short time before the link fails and is again renegotiated. If the affected port is a parallel uplink between switches, this fault can cause spanning-tree problems and loss of logical connectivity across a broadcast domain.