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Troubleshooting Interfaces

This topic discusses about various troubleshooting scenarios.

Troubleshooting: Faulty Ethernet Physical Interface on an M Series, an MX Series, or a T Series Router

You can follow the basic troubleshooting checklist as explained in the following topics from one through five to troubleshoot an Ethernet physical interface on an M Series, MX Series, or a T Series router.

Checking the Cable Connection

Problem

Description

Packets are not received or transmitted over the Ethernet physical interface.

Diagnosis

  1. Is the correct cable connected to the correct port?

Resolution

Resolving Cabling Issue

Perform one or more of the following steps to resolve the cabling issue:

  1. Connect the cable properly on the local and remote ends without any loose connections.

  2. Swap the Ethernet cable for a known good cable if the existing cable is damaged.

  3. Connect a single-mode fiber cable to a single-mode interface only and a multimode fiber cable to a multimode interface only. To check fiber optic cable integrity, see Checking Fiber Optic Cable Integrity.

  4. Connect the correct small form-factor pluggable transceiver (SFP) on both sides of the cable.

Checking Fiber Optic Cable Integrity

To check the integrity of fiber optic cable with an external cable diagnostic testing tool:

Note:

A single-mode fiber cable must be connected to a single-mode interface and a multi-mode fiber cable must be connected to a multi-mode interface.

  1. Measure the received light level at the receiver (RX) port to see whether the received light level is within the receiver specification of the Ethernet interface.

  2. Measure transmitted light level at the transmitter (TX) port to see whether the transmitted light level is within the transmitter specification of the Ethernet interface.

Checking the Physical Link Status of the Interface

Problem

Description

Unable to transmit and receive packets on the Ethernet interface even though the cable connection is correct.

Solution

To display the physical link status of the interface, run the show interface interface-name media operational mode command. For example, on the ge-5/0/1 interface.

For information about show interfaces interface-name media, see show interfaces.

Diagnosis

  1. Are there any connectivity problems such as input errors and packet loss even though the Enabled field displays Physical link is Up status and the Active alarms and Active defect field displays None?

  1. Does the Enabled field display Physical link is Down status and the Active alarms and Active defect field display Link?

    • Yes: The interface is either not connected correctly or is not receiving a valid signal. Go to Resolving Cabling Issue.

    • No: Continue.

Checking the Interface Statistics in Detail

Problem

Description

The physical interface is not working even though the Enabled field displays Physical link is Up status and the Active alarms and Active defect field displays None.

Solution

To display the interface statistics in detail, run the show interface interface-name extensive operational command. For example, on ge-5/0/1 interface.

For information about show interfaces interface-name detail, see show interfaces.

Diagnosis

  1. Does the Policed discards, L2 channel errors, Input DA rejects, or the Input SA rejects field display any errors?

    For information about the errors, seeshow interfaces.

Performing the Loopback Diagnostic Test

Problem

Description

The interface cable is connected correctly and there are no alarms or errors associated with the Ethernet physical interface yet the interface is not working.

Solution

To check whether the Ethernet port or PIC is faulty, you must perform the internal loopback test and hardware loopback test.

To perform a internal loopback diagnostic test on an Ethernet interface, for example on ge-5/0/1 interface:

  1. In configuration mode, go to the [edit interfaces ge-5/0/1] hierarchy level.

  2. Set the gigether-options option as loopback, commit the configuration and quit configuration mode.

  3. In operational mode, execute the show interfaces ge-5/0/1 media command.

Note:

Delete the loopback statement after completing your diagnosis.

Execute one of the following steps for a hardware loopback diagnostic test as needed:

  • For an Ethernet PIC with a fiber optic interface—Physically loop the TX and RX port and check the status of the physical link with the show interfaces interface-name media operational mode command.

  • For an Ethernet PIC with an RJ-45 Ethernet interface—Build a loopback plug by crossing pin 1 (TX +) to pin 3 (RX +) together and pin 2 (TX -) and pin 6 (RX -) together and check the status of the physical link with the show interfaces interface-name media operational mode command.

Note:

For information about loopback testing, see Performing Loopback Testing for Fast Ethernet and Gigabit Ethernet Interfaces.

Diagnosis

  1. Does the Enabled field display Physical link is Up status and the Active alarms and Active defect field display None when you perform the loopback test?

  1. When the Ethernet interface is connected to a remote Ethernet device over multiple patch panels, check to see whether the connection can be looped back at the different patch panels so you can conduct a loopback diagnostic test. Is the loopback diagnostic test successful?

Checking Other Possibilities

Problem

Description

Loopback diagnostic test is successful but unable to transmit and receive packets on the Ethernet interface.

Solution

Use the following commands as needed to troubleshoot an Ethernet interface, for example, an ge-5/0/1 interface:

  • Run the show interfaces interface-name terse operational command to check if the physical interface and logical interfaces are administratively disabled. For example, on ge-5/0/1 interface.

Diagnosis

  1. Does the physical interface and its corresponding logical interfaces display down in the output of the show interfaces interface-name terse operational mode command?

  1. Are the speed, duplex, and auto-negotiation fields in the output of show interfaces interface-name extensive operational mode command correctly set for the interface?

    Note:

    Check if the associated Flexible PIC Concentrator (FPC), Modular Port Concentrator (MPC), or Dense Port Concentrator (DPC) and its Modular Interface Card (MIC) or PIC with its 10-gigabit small form-factor pluggable transceiver (XFP) or SFP supports speed and auto-negotiation settings.

To Enable a Physical Interface

To enable a physical interface:

  1. In configuration mode, go to the [edit interfaces] hierarchy level.
  2. Check if the interface is administratively disabled by executing the show command on the interface. For example on ge-5/0/1 interface.
  3. Enable the interface and commit.

Time Domain Reflectometry on ACX Series Routers Overview

Time Domain Reflectometry (TDR) is a technology used for diagnosing copper cable states. This technique can be 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 the 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.

Diagnosing a Faulty Twisted-Pair Cable on ACX Series Routers

Problem

Description

A 10/100BASE-T Ethernet interface has connectivity problems that you suspect might be caused by a faulty cable.

Solution

Use the time domain reflectometry (TDR) test to determine whether a twisted-pair Ethernet cable is faulty.

The TDR test:

  • Detects and reports faults for each twisted pair in an Ethernet cable. Faults detected include open circuits, short circuits, and impedance mismatches.

  • Reports the distance to fault to within 1 meter.

  • Detects and reports pair swaps, pair polarity reversals, and excessive pair skew.

The TDR test is supported on the following ACX routers and interfaces:

  • 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.

Note:

We recommend running the TDR test on an interface when there is no traffic on the interface.

TDR diagnostics are applicable for copper ports only and not for optical fiber ports.

To diagnose a cable problem by running the TDR test:

  1. Run the request diagnostics tdr command.

  2. View the results of the TDR test with the show diagnostics tdr command.

  3. Examine the Cable status field for the four MDI pairs to determine if the cable has a fault. In the preceding example, the twisted pair on pins 4 and 5 is broken or cut at approximately one meter from the ge-0/0/10 port connection.

Note:

The Test Status field indicates the status of the TDR test, not the cable. The value Passed means the test completed—it does not mean that the cable has no faults.

The following is additional information about the TDR test:

  • The TDR test can take some seconds to complete. If the test is still running when you execute the show diagnostics tdr command, the Test status field displays Started. For example:

  • You can terminate a running TDR test before it completes by using the request diagnostics tdr abort interface interface-name command. The test terminates with no results, and the results from any previous test are cleared.

  • You can display summary information about the last TDR test results for all interfaces on the router that support the TDR test by not specifying an interface name with the show diagnostics tdr command. For example: