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Using Loopback Testing for SONET Interfaces

 

This section includes the following information to assist you when troubleshooting SONET interfaces:

Checklist for Using Loopback Testing for SONET Interfaces

Purpose

To use loopback testing to isolate SONET interface problems.

Action

Table 1 provides the links and commands for using loopback testing for SONET interfaces.

Table 1: Checklist for Using Loopback Testing for SONET Interfaces

Tasks

Command or Action

Diagnosing a Suspected Hardware Problem with a SONET Interface
  1. Creating a Loopback

Create a loopback.

Connect the transmit port to the receive port.

[edit interfaces interface-namesonet-options]

set loopback local

show

commit

  1. Setting Clocking to Internal

[edit interfaces interface-name]

set clocking internal

show

commit

  1. Verifying That the SONET Interface Is Up

show interfaces so-fpc/pic/port

  1. Clearing SONET Interface Statistics

clear interfaces statistics so-fpc/pic/port

  1. Checking That the Received and Transmitted Path Trace Are the Same

show interfaces so-fpc/pic/port extensive

  1. Forcing the Link Layer to Stay Up

 

[edit interfaces interface-name]

set encapsulation cisco-hdlc

show

commit

[edit interfaces interface-name]

set no-keepalives

show

commit

  1. Verifying the Status of the Logical Interface

show interfaces so-fpc/pic/port show interfaces so-fpc/pic/port terse

  1. Pinging the SONET Interface

ping interface so-fpc/pic/port local-IP-address bypass-routing count 1000 rapid

  1. Checking for SONET Interface Error Statistics

show interfaces so-fpc/pic/port extensive

Diagnosing a Suspected Circuit Problem
  1. Creating a Loop from the Router to the Network

[edit interfaces interface-name sonet-options]

set loopback remote

show

commit

  1. Creating a Loop to the Router from Various Points in the Network

Perform Steps 2 through 8 from Diagnosing a Suspected Hardware Problem with a SONET Interface.

Diagnosing a Suspected Hardware Problem with a SONET Interface

Problem

Description: When you suspect a hardware problem, take the following steps to verify if there is a problem.

Solution

To diagnose a suspected hardware problem with the SONET interface, follow these steps:

Creating a Loopback

You can create a physical loopback or configure a local loopback to help diagnose a suspected hardware problem. Creating a physical loopback is recommended because it allows you to test and verify the transmit and receive ports. If a field engineer is not available to create the physical loopback, you can configure a local loopback for the interface. The local loopback creates a loopback internally in the Physical Interface Card (PIC).

  1. Creating a Physical Loopback

  2. Configuring a Local Loopback



Creating a Physical Loopback

Action

To create a physical loopback at the port, connect the transmit port to the receive port using a known good fiber cable.

Note

Make sure you use a single-mode fiber for a single-mode port and multimode fiber for a multimode port. (For OC192, you must use the appropriate attentuation.)

Meaning

When you create and test a physical loopback, you are testing the transmit and receive ports of the PIC. This action is recommended if a field engineer is available to create the physical loop as it provides a more complete test of the PIC.



Configuring a Local Loopback

Action

To configure a local loopback without physically connecting the transmit port to the receive port, follow these steps:

  1. In configuration mode, go to the following hierarchy level.

  2. Configure the local loopback option.

  3. Verify the configuration.

  4. Commit the change.

Meaning

When you create a local loopback, you create an internal loop on the interface being tested. A local loopback loops the traffic internally on that PIC. A local loopback tests the interconnection of the PIC but does not test the transmit and receive ports.

Note

Remember to delete the loopback statement after completing the test.

Setting Clocking to Internal

Purpose

Clocking is set to internal because there is no external clock source in a loopback connection.

Action

To configure clocking to internal, follow these steps:

  1. In configuration mode, go to the following hierarchy level.

  2. Configure clocking to internal.

  3. Verify the configuration.

  4. Commit the change.

Meaning

The clock source for the interface is set to the internal Stratum 3 clock.

Verifying That the SONET Interface Is Up

Purpose

Displaying the status of the SONET interface provides the information you need to determine whether the physical link is up or down.

Action

To verify that the SONET interface is up, use the following Junos OS command-line interface (CLI) operational mode command:

Sample Output 1

The following output is for a SONET interface with the physical link up:

Sample Output 2

When you see that the physical link is down, there might be a problem with the port. Sample output 2 shows that the physical link is down:

Meaning

Sample output 1 shows that the physical link is up, the loop is detected, and there are no SONET alarms or defects.

If the physical link is up, continue with Checking That the Received and Transmitted Path Trace Are the Same.

The sample output 2 shows that the physical link is down, the device flags and interface flags are down, and there are SONET alarms and defects.

Table 2 lists problem situations and actions for a physical link that is down.

Table 2: Problems and Solutions for a Physical Link That Is Down

Problem

Action

Cable mismatch

Verify that the fiber connection is correct.

Damaged and/or dirty cable

Verify that the fiber can successfully loop a known good port of the same type.

Too much or too little optical attenuation

Verify that the attenuation is correct per the PIC optical specifications.

The transmit port is not transmitting within the dBm optical range per the specifications

Verify that the Tx power of the optics is within range of the PIC optical specification.

Clearing SONET Interface Statistics

Purpose

You must reset SONET interface statistics before you initiate the ping test. Resetting the statistics provides a clean start so that previous input/output errors and packet statistics do not interfere with the current diagnostics.

Action

To clear all statistics for the interface, use the following Junos OS CLI operational mode command:

user@host> clear interfaces statistics so-fpc/pic/port

Sample Output

Meaning

This command clears the interface statistics counters for interface so-4/0/2 only.

Checking That the Received and Transmitted Path Trace Are the Same

Purpose

The received and transmitted path trace shows whether the transmitted path trace is looped back.

Action

To check that the received path trace matches the transmitted path trace, use the following Junos OS CLI operational mode command:

Sample Output

user@host# show interfaces so-2/2/0 extensive

Meaning

This transmitted and received path trace information is near the end of the output. The sample output shows that the transmitted and received path trace are the same. When there is a loopback, the transmitted and received path trace should be the same. If they are, continue with Forcing the Link Layer to Stay Up.

If the transmitted and received path trace are not the same, the physical loopback cable is probably on the wrong port, or is incorrectly connected. In this case, verify the connection again.

Forcing the Link Layer to Stay Up

To complete the loopback test, the link layer must remain up. However, Junos OS is designed to recognize that loop connections are not valid connections and to bring the link layer down. You need to force the link layer to stay up by making some configuration changes to the encapsulation and keepalives.

To force the link layer to stay up, follow these steps:

  1. Configuring Encapsulation to Cisco-HDLC

  2. Configuring No-Keepalives



Configuring Encapsulation to Cisco-HDLC

Action

To configure encapsulation on a SONET physical interface, follow these steps:

  1. In configuration mode, go to the following hierarchy level.

  2. Configure Cisco-HDLC encapsulation on the interface.

  3. Verify the configuration.

  4. Commit the change.

Meaning

This command sets the interface encapsulation to the Cisco High-level Data-Link Control (HDLC) transport protocol.



Configuring No-Keepalives

Action

To disable the sending of link-layer keepalives on a SONET physical interface, follow these steps:

  1. In configuration mode, go to the following hierarchy level.

  2. Configure the no-keepalives statement.

  3. Verify the configuration.

  4. Commit the change.

Meaning

By setting the no-keepalives statement, the link layer is forced to stay up. If the setting remains at keepalive, the router will recognize that the same link-layer keepalives are being looped back and will bring the link layer down.

Pinging the SONET Interface

Purpose

To ping the local interface and verify the loopback connection, use the following Junos OS CLI operational mode command:

Action

Sample Output

user@host# ping interface so-2/2/0 10.0.2.1 bypass-routing count 1000 rapid

PING 10.0.2.1 (10.0.2.1): 56 data bytes

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Meaning

This command sends 1000 ping packets out of the interface to the local IP address. The ping should complete successfully with no packet loss. If there is any persistent packet loss, open a case with the Juniper Networks Technical Assistance Center (JTAC) at support@juniper.net or at 1-888-314-JTAC (within the United States) or 1-408-745-9500 (from outside the United States).

Checking for SONET Interface Error Statistics

Purpose

Persistent interface error statistics indicate that you need to open a case with JTAC.

Action

To check the local interface for error statistics, use the following Junos OS CLI operational mode command:

Sample Output

user@host# show interfaces so-2/2/0 extensive

Meaning

Check for any error statistics that may appear in the section, line, and path areas of the output. There should not be any input or output errors. If there are any persistent input or output errors, open a case with JTAC at support@juniper.net or at 1-888-314-JTAC (within the United States) or 1-408-745-9500 (from outside the United States).

Diagnosing a Suspected Circuit Problem

When you suspect a circuit problem, it is important to work with the transport-layer engineer to resolve the problem. The transport-layer engineer may ask you to create a loop from the router to the network, or the engineer may create a loop to the router from various points in the network.

To diagnose a suspected circuit problem, follow these steps:

  1. Creating a Loop from the Router to the Network

  2. Creating a Loop to the Router from Various Points in the Network



Creating a Loop from the Router to the Network

Purpose

Creating a loop from the router to the network allows the transport-layer engineer to test the router from various points in the network. This helps the engineer isolate where the problem might be located.

Action

To create a loop from the router to the network, follow these steps:

  1. In configuration mode, go to the following hierarchy level.

  2. Configure the remote loopback option.

  3. Verify the configuration.

  4. Commit the change.

Meaning

This command loops any traffic from the network back into the network.



Creating a Loop to the Router from Various Points in the Network

Purpose

The transport-layer engineer creates a loop to the router from various points in the network. You can then perform tests to verify the connection from the router to that loopback in the network.

Action

After the transport-layer engineer has created the loop to the router from the network, you must verify the connection from the router to the loopback in the network. Follow Steps 2 through 8 in Diagnosing a Suspected Hardware Problem with a SONET Interface. Keep in mind that any problems encountered in the test indicate a problem with the connection from the router to the loopback in the network.

By performing tests to loopbacks at various points in the network, you can isolate the source of the problem.