The bucket of cable problems contains issues such as wrong cables, broken cables, and incorrectly connected cables. Too often, administrators overlook cables as a trouble source. This is especially true whenever the system "was working." This causes troubleshooters to assume that because it was working, it must still be working. They then investigate other problem areas, only to return to cables after much frustration.
Common cable mistakes during installation generally include using the wrong cable type. One, for example, is the use of a crossover cable rather than a straight through cable, or vice versa. The following list summarizes many of the typical problems:
• Crossover rather than straight through, or vice versa
• Single-mode rather than multimode
• Connecting transmit to transmit
• Connecting to the wrong port
• Partially functional cables
• Cable works in simplex mode, but not full-duplex
• Cables too long or too short for the media
Remember that when attaching an MDI (media dependent interface) port to an MDIX (media dependent crossover interface) port, you must use a straight through cable. All other comb inations require a crossover cable type. Fortunately, using the wrong cable type keeps the link status light extinguished on equipment. This provides a clue that the cable needs to be examined. An extinguished link status light can result from the correct cable type, but a broken one. Be aware that an illuminated link status light does not guarantee that the cable is good either. The most that you can conclude from a status light is that you have the correct cable type and that both pieces of equipment detect each other. This does not, however, mean the cable is capable of passing data.
A form of partial cable failure can confuse some network operations like Spanning Tree. For example, if your cable works well in one direction, but not the other, your Catalyst might successfully transmit BPDUs, but not receive them. When this happens, the converged Spanning Tree topology might be incorrect and, therefore, dysfunctional.
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