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Use Loopback Testing for T1 Interfaces

 

This section includes the following information to assist you when troubleshooting T1 interfaces:

Checklist for Using Loopback Testing for T1 Interfaces

Purpose

To use loopback testing for T1 interfaces.

Action

Table 1 provides commands for using loopback testing for T1 interfaces.

Table 1: Checklist for Using Loopback Testing for T1 Interfaces

Tasks

Command or Action

Diagnose a Suspected Hardware Problem with a T1 Interface
  1. Create a Loopback

 

Connect a T1 loopback plug.

[edit interfaces interface-name t1-options]

set loopback local

show

commit

  1. Set Clocking to Internal

[edit interfaces interface-name]

set clocking internal

show

commit

  1. Verify That the T1 Interface Is Up

show interfaces t1-fpc/pic/port

  1. Clear T1 Interface Statistics

clear interfaces statistics t1-fpc/pic/port

  1. Force 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. Verify the Status of the Logical Interface

show interfaces t1-fpc/pic/port

show interfaces t1-fpc/pic/port terse

  1. Ping the T1 Interface

ping interface t1-fpc/pic/port local-IP-address bypass-routing count 1000 rapid

  1. Check for T1 Interface Error Statistics

show interfaces t1-fpc/pic/port extensive

Diagnose a Suspected Circuit Problem
  1. Create a Loop from the Router to the Network

[edit interfaces interface-name t1-options]

set loopback remote

show

commit

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

Perform Steps 2 through 8 from Diagnose a Suspected Hardware Problem with a T1 Interface.

Diagnose a Suspected Hardware Problem with a T1 Interface

Problem

Description: Take the following steps to verify if there is a hardware problem with a T1 interface.

Solution

To diagnose a suspected hardware problem with a T1 interface, follow these steps:

  1. Create a Loopback

  2. Set Clocking to Internal

  3. Verify That the T1 Interface Is Up

  4. Clear T1 Interface Statistics

  5. Force the Link Layer To Stay Up

  6. Verify the Status of the Logical Interface

  7. Ping the T1 Interface

  8. Check for T1 Interface Error Statistics

Create 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 T1 port. 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. Create a Physical Loopback

  2. Configure a Local Loopback



Create a Physical Loopback

Action

To create a physical loopback at the T1 port, connect a T1 loopback plug to the T1 port. You can make a T1 loopback plug by connecting pin 1 to pin 4 and pin 2 to pin 5 on an RJ-48 plug.

Meaning

When you create and test a physical loopback, you are testing the T1 port. 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.



Configure 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 loopback:

  3. Verify the configuration:

  4. Commit the change:

    For example:

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.

Set Clocking to Internal

Purpose

You set clocking 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 the clocking to internal:

  3. Verify the configuration:

    For example:

  4. Commit the change:

    For example:

Meaning

This command saves the clocking change to the configuration database, activates the configuration on the router, and exits configuration mode.

Verify That the T1 Interface Is Up

Purpose

Display the status of the T1 interface to determine whether the physical link is up or down.

Action

To verify that the status of the T1 interface is up, use the following Junos OS command-line interface (CLI) operational mode command:

Sample Output

The following output is for a T1 interface with the physical link up:

user@host> show interfaces t1-1/1/0

Meaning

The sample output shows that the physical link is up, the loop is detected, and there are no T1 alarms or defects.

Sample Output

If the physical link is down, there may be a problem with the port. The following output is an example of the show interfaces t1-fpc/pic/port command when the physical link is down:

user@host> show interfaces t1-1/1/0

Meaning

The sample output shows that the physical link is down, the device flags and interface flags are down, and that there are T1 alarms and defects. Verify that the fiber can successfully loop a known good port of the same type by checking for damage to the cable.

Clear T1 Interface Statistics

Purpose

You must reset T1 interface statistics before initiating 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:

Sample Output

user@host> clear interfaces statistics t1-1/1/0

Meaning

This command clears the interface statistics counters for interface t1-1/1/0 only.

Force 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. Configure Encapsulation to Cisco-HDLC

  2. Configure No-Keepalives

Configure Encapsulation to Cisco-HDLC

Action

To configure encapsulation on a T1 physical interface, follow these steps:

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

  2. Configure encapsulation to Cisco-HDLC:

  3. Verify the configuration:

    For example:

  4. Commit the change:

    For example:

Meaning

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

Configure No-Keepalives

Action

To disable the sending of link-layer keepalives on a T1 physical interface, follow these steps:

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

  2. Configure no-keepalives:

    For example:

  3. Verify the configuration:

    For example:

  4. Commit the change:

    For example:

Meaning

By setting no-keepalives, 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.

Verify the Status of the Logical Interface

Purpose

To verify the status of the logical interface, use the following two Junos OS CLI operational mode commands:

Action

Sample Output

The following output is for a logical interface that is up:

user@host> show interfaces t1-1/1/0

Meaning

The sample output for the first command shows that the logical link is up because there are no flags indicating that the link layer is down. The output for the show interfaces terse command shows that logical interface t1-1/0/0 is up.

Sample Output

The following output is for a logical interface that is down:

user@host> show interfaces t1-1/1/0

Meaning

The sample output for both commands shows that the logical interface is down. The first command shows that the link layer, device, and destination route are all down. The second command shows that logical interface t1-1/1/0.0 is down.

Ping the T1 Interface

Purpose

Use the ping command to verify the loopback connection.

Action

To ping the local interface, use the following Junos OS CLI operational mode command:

Sample Output

user@host> ping interface t1-1/1/0 1.1.1.1 bypass-routing count 1000 rapid

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).

Check for T1 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 t1-1/1/0 extensive

Meaning

Check for any error statistics that may appear in 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).

Diagnose 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. Create a Loop from the Router to the Network

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



Create 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 is 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 remote loopback:

  3. Verify the configuration:

    For example:

  4. Commit the change:

    For example:

Meaning

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



Create 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 Step 2 through Step 8 in Diagnose a Suspected Hardware Problem with a T1 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.

Related Documentation