Troubleshooting Interfaces

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Time Domain Reflectometry (TDR) is a technology used for diagnosing copper cable states. This technique is used to determine if cabling is at fault when you cannot establish a link. TDR detects the defects by sending a signal through a cable, and reflecting it from the end of the cable. Open circuits, short circuits, sharp bends, and other defects in the cable, reflects the signal back, at different amplitudes, depending on the severity of the defect.

Several factors that result in degraded or low-quality cable plants can cause packet loss, suboptimal connection speed, reduced network efficiency, and complete connection failures. These types of problems can occur because of poor cable construction, identification of pair twists, loose connectors, poor contacts between the points, and stretched or broken pairs of cables. Broadcom transceivers enable you to analyze the condition of the cable plant or topology and identify any problems that have occurred. This functionality is effectively used in the following scenarios:

• Troubleshooting during initial network equipment installation.

• Discovery of failures when network problems occur.

• Maintenance of optimally functioning cable plants.

• Fault determination during the testing of network equipment in production cable networks.

TDR supports the following capabilities for examination of cable faults on ACX Series routers:

• Cable status pair (open or short)—When the router operates in Gigabit Ethernet mode, all four pairs (8 wires) are used. Only Pair-A and Pair-B are required to operate in 10/100BASE-T Ethernet mode. If either of these required pairs is open or short-circuited, the transceiver reports the following faults:

  • Any open wire

  • Wires of a particular pair that are shorted

• Distance to fault per pair—Distance at which an open or a short-circuit is detected in meters. This measurement is also termed as cable length. The transceiver reports the following faults:

  • Cable length when the cable status is normal

  • Distance to fault when the cable status is not normal

• Pair Swap—Swapping of twisted-pairs in straight-through and cross-over cable plants are detected.

• Polarity Swap—Each cable pair carries a differential signal from one end to the other end of the cable. Each wire within the pair is assigned a polarity. The wires in a pair are normally connected in a one-to-one form. This connection enables the transmitter at one end to be connected to the receiver at the other end with same polarity. Sometimes, the wiring within the pair is also swapped. This type of connection is called polarity swap. Broadcom transceivers can detect such swapping and automatically adjust the connection to enable the links to operate normally. However, the transceiver reports polarity swaps that it detects in the cable plant.

On 4-port Gigabit Ethernet and 8-port Gigabit Ethernet MICs with copper SFP transceivers (using BCM54880) and 4-port Gigabit Ethernet, 6-port Gigabit Ethernet, and 8-port Gigabit Ethernet MICs with copper and optical SFP transceivers (using BCM54640E PHY), only 10BASE-T pair polarity is supported. 100BASE-T and 1000BASE-T polarities are not supported.

When the Gigabit Ethernet link cannot be established (for example, if only two pairs are present that are fully functional), TDR in the physical layer (PHY) brings down the link to a 100 MB link, which is called a downshift in the link. The physical layer might require 10-20 seconds for the link to come up if a downgrade in wire speed occurs because it attempts to connect at 1000 MB five times before it falls back to 100BASE-TX.

TDR diagnostics is supported only on copper interfaces and not on fiber interfaces.

Keep the following points in mind when you configure TDR:

• If you connect a port undergoing a TDR test to a Gigabit Ethernet interface that is enabled to automatically detect MDI (Media Dependent Interface) and MDIX (Media Dependent Interface with Crossover) port connections, the TDR result might be invalid.

• If you connect a port undergoing a TDR test to a 100BASE-T copper interface, the unused pairs are reported as faulty because the remote end does not terminate these pairs.

• You must not modify the port configuration while the TDR test is running.

• Because of cable characteristics, you need to run the TDR test multiple times to get accurate results.

• Do not change the port status (such as removing the cable at the near or far end) because such a change can result in inaccurate statistics in the results.

• While measuring the cable length or distance to fault (per pair), sometimes, a few cable length inconsistencies might be observed during a TDR test. Broadcom transceivers have the following cable length limitations:

  • For a properly-terminated good cable, the accuracy of the cable length reported is plus or minus 10 meters.

  • If a pair is open or short-circuited, the far-end termination does not affect the computed result for that pair.

  • The accuracy of the measured cable length, when open and short-circuit conditions are detected, is plus or minus 5 meters.

  • The accuracy of a good pair, when one or more pairs are open or short-circuited, is plus or minus 10 meters.

• Polarity swap detection is supported only in 10BASE-T mode.

• The TDR test does not impact the traffic if the interface operates at 10-Gigabit Ethernet per second of bandwidth, which is the default configuration. However, if the speed of the interface is configured to be other than 10-Gigabit Ethernet, running the TDR test affects the traffic.

  TDR diagnostics might bring the link down and initialize the physical layer (PHY) with default configuration to perform its operation.

  When the TDR validation test is completed, the PHY layer resumes operation in the same manner as before the cable diagnostics test was performed. However, link flaps might be momentarily observed. We recommend that you run the TDR test at a speed of 1 gigabit per second, which is the default configuration, to obtain more accurate results.

TDR is supported on the following interfaces on ACX Series routers:

• On ACX1000 routers, 4 RJ45 (Cu) ports or 8-port Gigabit Ethernet MICs with small form-factor pluggable (SFP) transceivers and RJ45 connectors.

  On ACX1100 routers, 4-port or 8-port Gigabit Ethernet MICs with SFP transceivers and RJ45 connectors.

• On ACX2000 routers, 8-port Gigabit Ethernet MICs with SFP transceivers and RJ45 connectors.

• On ACX2100 and ACX2200 routers, 4-port Gigabit Ethernet MICs with SFP transceivers and RJ45 connectors.

• On ACX4000 routers, 4-port, 6-port, or 8-port Gigabit Ethernet MICs with SFP transceivers and RJ45 connectors.

You must select the media type as copper for the 1-Gigabit Ethernet interfaces. To specify the media type, include the media-type statement with the copper option at the [edit interfaces interface-name] hierarchy level. Media type selection is applicable to ports only in slot 2. When media-type is not set, the port accepts either type of connection. The media type is fiber if a transceiver is installed in the SFP connection. If no transceiver is installed, the media type is copper. The COMBO ports (combination ports) on ACX routers support both the copper and fiber-optic media types. On such ports or interfaces, you must configure the media type as copper to run the TDR test.

You can run the TDR test from operational mode and view the success or failure results of the test. To start a test on a specific interface, issue the request diagnostics tdr start interface interface-name command. To stop the TDR test currently in progress on the specified interface, issue the request diagnostics tdr abort interface interface-name command. To display the test results for all copper interfaces, enter the show diagnostics tdr command. To display the test results for a particular interface, enter the show diagnostics tdr interface interface-name command.

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