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Verify Link and Transceivers with Pseudo Random Binary Sequence (PRBS) Test

date_range 20-Dec-24

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Learn how to verify links and transceivers using the Pseudo Random Binary Sequence (PRBS) test and perform bidirectional diagnostics with remote loopback. Discover essential steps for clearing interface statistics effectively.

How to Use Pseudo Random Binary Sequence (PRBS) Test for Link and Transceivers Verification

The Pseudo Random Binary Sequence (PRBS) test enables you to verify link quality and transceiver operation. There are two diagnostic scenarios:

Bidirectional verification: The test sequence starts from the local end, and the remote end is configured with loopback. Analyze the test pattern at both ends.
Unidirectional verification: The test sequence starts from the local end, and the remote end analyzes the test pattern.

In the first case, bidirectional verification requires loopback support on the remote end.

Table 1: PRBS test on various MICs:
MIC Type	PRBS Test Engine	Description
JNP10003-LC2103	Packet Forwarding Engine (PFE)	The PFE for JNP10003-LC2103 is a specialized hardware module designed to handle high-performance packet forwarding, routing, and traffic management for the Juniper Networks JNP10003 system.
JNP-MIC1	Packet Forwarding Engine (PFE)	The PFE for JNP-MIC1 is a hardware module designed to manage and accelerate packet forwarding, enabling high-speed data processing and routing for the Juniper Networks JNP Series Modular Interface Cards (MIC).
JNP-MIC1-MACSEC	External physical layer (PHY) device	External physical layer (PHY) device is not capable of passing the PRBS pattern originating from the packet forwarding engine. Hence, the PRBS test is enabled on external physical layer (PHY) device.

Table 2: PRBS test details supported on various interfaces:
Interface Type	Interface Name	Interface Lane Characteristics	Description
10-Gigabit ethernet interface	xe	The interface is supported with 1 lane of 10Gbps speed	The PRBS test is executed on each lane supported. Hence, the `show interfaces prbs-stats` displays data for one lane.
40-Gigabit ethernet interface	et	The interface is supported with 4 lanes of 10Gbps speed.	The PRBS test is executed on each lane supported. Hence, the `show interfaces prbs-stats` displays data for four lanes.
100-Gigabit ethernet interface	et	The interface is supported with 4 lanes of 25Gbps speed.	The PRBS test is executed on each lane supported. Hence, the `show interfaces prbs-stats` displays data for four lanes.

Configure Unidirectional Diagnostics

Enable the transmission of pseudo-random binary sequence pattern on one end of the link (TX), and enable monitoring on the other end (RX).
The PRBS statistics report on the receiving end reflects link quality.
Collect and view the PRBS statistics.
1. Start a TX (direction 0) by issuing the following command:
```
[edit ]
user@host1> test interface et-0/1/2 prbs-test-start pattern-type 31 direction 0 flip 0
```
  Check the link status by executing show interfaces terse et-0/1/*:
```
 Interface               Admin Link Proto    Local                 Remote
 et-0/1/2                up    down
```
2. Start a RX (direction 1) by issuing the following command:
```
[edit ]
user@host2> test interface et-1/1/4 prbs-test-start pattern-type 31 direction 1 flip 0
```
  Check the link status by executing show interfaces terse et-1/1/4:
```
Interface               Admin   Link Proto    Local                 Remote
et-1/1/4             up    down
```
3. View the collected statistics at RX by issuing the following command:
```
[edit ]
user@host2> show interfaces interface-name prbs-stats
```
  For example:
  
  Checking PRBS statistics at RX:
```
user@host2> show interfaces et-1/1/4 prbs-stats
PRBS Statistics : Enabled 

Lane 0 : State : Pass, Error count : 0
Lane 1 : State : Pass, Error count : 0
Lane 2 : State : Pass, Error count : 0
Lane 3 : State : Pass, Error count : 0
```
  The PRBS test is successful, if the state is pass with error count 0.
4. Stop the PRBS statistics collection by issuing the following command:
```
user@host2> test interface interface-name prbs-test-stop direction 1 
user@host1> test interface interface-name prbs-test-stop direction 0 
```
  For example:
  
  To stop the PRBS at RX:
```
user@host2> test interface et-1/1/4 prbs-test-stop direction 1
```
  Verify the statistics at TX by executing the following command:
```
show interfaces et-0/1/2 prbs-stats
PRBS Statistics : Disabled
```
  To stop the PRBS at TX:
```
user@host1> test interface et-0/1/2 prbs-test-stop direction 0
```
  After executing the command, you can check the link status at RX by executing show interfaces terse et-1/1/4:
```
Interface               Admin Link Proto    Local                 Remote
et-1/1/4                up    up
```
  Check the link status at TX by executing show interfaces terse et-0/1/2:
```
Interface               Admin Link Proto    Local                 Remote
et-0/1/2                up    up
```
  This command only disables the statistics collection and does not clear the statistics collected. To clear the collected statistics, issue the clear interfaces statistics command.
```
user@host1> clear interfaces statistics et-0/1/2
```

Configure Bidirectional Diagnostics with Remote Loopback

Configure loopback on the remote end of the link. Test pattern generation and analysis is performed on the local end.

The PRBS statistics report on the receiving end reflects link quality.

Following are the steps to collect and view the PRBS statistics:

Enable remote loopback.
```
user@host2> set interfaces et-1/1/4 gigether-options loopback-remote 
```
You must disable Forward Error Correction (FEC), if loopback is configured in the router with JNP-MIC1 MIC at the remote end.
```
user@host2> show interfaces et-1/1/4 | display set 
set interfaces et-1/1/4 gigether-options fec none
```

Start a TX (direction 0) by issuing the following command:

content_copy zoom_out_map
[edit]
user@host1> test interface et-0/1/2 prbs-test-start pattern-type 31 direction 0 flip 0 

After executing the command, you can check the link status by executing show interfaces terse et-0/1/*:

content_copy zoom_out_map
Interface	Admin    Link Proto	Local	Remote
et-0/1/2	up	      down	                                    

Start a RX (direction 1) by issuing the following command on the same host.
```
[edit]
user@host1> test interface et-0/1/2 prbs-test-start pattern-type 31 direction 1 flip 0
```

View the collected statistics at RX by issuing the following command:

content_copy zoom_out_map
[edit]
user@host1> show interfaces interface-name prbs-stats

For example:

Checking PRBS statistics at RX:

content_copy zoom_out_map
user@host1> show interfaces et-0/1/2 prbs-stats
PRBS Statistics : Enabled 

Lane 0 : State : Pass, Error count : 0 
Lane 1 : State : Pass, Error count : 0 
Lane 2 : State : Pass, Error count : 0 
Lane 3 : State : Pass, Error count : 0

The PRBS test is successful, if the state is pass with error count 0.

Stop the PRBS statistics collection by issuing the following command:
```
user@host1> test interface interface-name prbs-test-stop direction  1
user@host1> test interface interface-name prbs-test-stop direction 0
```
For example:

To stop the PRBS at RX:
```
user@host1> test interface et-0/1/2 prbs-test-stop direction 1
```
Verify the statistics at TX by executing the following command:
```
show interfaces et-0/1/2 prbs-stats
PRBS Statistics : Disabled
```
To stop the PRBS at TX:
```
user@host1> test interface et-0/1/2 prbs-test-stop direction 0
```
After executing the command, you can check the link status at RX by executing show interfaces terse et-0/1/2:
```
Interface	Admin    Link Proto	Local	Remote
et-0/1/2	up	      up	
```
This command only disables the statistics collection and does not clear the statistics collected. To clear the collected statistics, issue the clear interfaces statistics command.
```
user@host1> clear interfaces statistics et-0/1/2
```

Table 3: Interface Card Specific Differences
Behavior	JNP-MIC1-MACSEC MIC	JNP-MIC1 MIC
RX Latching to PRBS Signal	If RX is not latching to any PRBS signal, then the "state" in the `show interfaces interface-name prbs-stats` displays as “Disabled" with Error count as 0. user@host> `test interface et-0/1/10 prbs-test-start pattern-type 31 direction 1 flip 0` user@host> `show interfaces et-0/1/10 prbs-stats` PRBS Statistics : Enabled Lane 0 : State : Disabled, Error count : 0 Lane 1 : State : Disabled, Error count : 0 Lane 2 : State : Disabled, Error count : 0 Lane 3 : State : Disabled, Error count : 0	If RX is not latching to any PRBS signal, then the "state" in the `show interfaces interface-name prbs-stats` displays as failed with MAX error count. user@host> `test interface et-0/0/1 prbs-test-start pattern-type 31 direction 1 flip 0` user@host> `show interfaces et-0/0/1 prbs-stats` PRBS Statistics : Enabled Lane 0 : State : Fail, Error count : 4294967295 Lane 1 : State : Fail, Error count : 4294967295 Lane 2 : State : Fail, Error count : 4294967295 Lane 3 : State : Fail, Error count : 4294967295
Pattern Mismatch	Displays a “disabled” state for pattern mismatches and flips between TX and RX (flip supported).	Maximum errors due to pattern mismatches between TX and RX flips (flip not supported).
TX Interruptions	RX shows “Disabled” with Error Count 0; no need to restart RX when TX restarts.	RX shows a failed state with error counts; RX must be restarted if TX restarts.
Consecutive Starts	Behaves differently from JNP-MIC1 MIC when starting TX or RX consecutively without stopping earlier runs.	Shows distinct behavior when starting TX or RX consecutively without stopping earlier runs.
DFE Tuning	Does not require Decision Feedback Equalization (DFE) tuning.	Requires DFE tuning; will show errors if PRBS restarts without stopping earlier runs.
Forward Error Correction (FEC	No specific mention of FEC settings.	FEC must be disabled if loopback is configured at the remote end. user@host> `show interfaces et-1/1/1 \| display set` `set interfaces et-1/1/1 gigether-options loopback-remote` `set interfaces et-1/1/1 gigether-options fec none`

Clear the Interface Statistics

The clear interface statistics command clears only the error counters and not the status, RX needs to be restarted to get the right status.

Following are the steps to clear the interface statistics:

Check the statistics at RX by issuing the following command:

content_copy zoom_out_map
[edit]
user@host2> show interfaces et-1/1/4 prbs-stats
PRBS Statistics : Enabled
    Lane 0 : State : Fail, Error count : 4294967295
    Lane 1 : State : Fail, Error count : 4294967295
    Lane 2 : State : Fail, Error count : 4294967295
    Lane 3 : State : Fail, Error count : 4294967295

Because only RX is started, there is no PRBS pattern and RX shows maximum error.

To clear the interface statistics, execute the following command:

content_copy zoom_out_map
[edit]
user@host2> clear interfaces statistics et-1/1/4 
user@host2> show interfaces et-1/1/4 prbs-stats
PRBS Statistics : Enabled
    Lane 0 : State : Fail, Error count : 0
    Lane 1 : State : Fail, Error count : 0
    Lane 2 : State : Fail, Error count : 0
    Lane 3 : State : Fail, Error count : 0

Here the status still shows as fail, although the statistics displays the delta value. In this case, because both the current and previous vales are INT_MAX, the delta value 0 is displayed.

Consider a scenario where the error count increments as the PRBS test is ongoing. In this case, the show interfaces interface-name prbs-stats shows the incremental error (delta value). Also after issuing clear interfaces statistics et-1/1/4, if the error count is updated, the clear interfaces statistics displays the incremental error too.

For Example:

content_copy zoom_out_map
[edit]
user@host2> show interfaces et-1/1/4 prbs-stats
PRBS Statistics : Enabled
    Lane 0 : State : Fail, Error count : 640
    Lane 1 : State : Fail, Error count : 647
    Lane 2 : State : Fail, Error count : 661
    Lane 3 : State : Fail, Error count : 596

[edit]
user@host2> show interfaces et-1/1/4 prbs-stats
PRBS Statistics : Enabled
    Lane 0 : State : Fail, Error count : 52
    Lane 1 : State : Fail, Error count : 65
    Lane 2 : State : Fail, Error count : 626
    Lane 3 : State : Fail, Error count : 132

As shown above, in the second instance the show interfaces interface-name prbs-stats command displays the "change" in error count. So, the total error count is 640 + 52 = 692 for Lane 0.

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Ethernet Interfaces User Guide for Routing Devices

ON THIS PAGE

Verify Link and Transceivers with Pseudo Random Binary Sequence (PRBS) Test

How to Use Pseudo Random Binary Sequence (PRBS) Test for Link and Transceivers Verification

Configure Unidirectional Diagnostics

Configure Bidirectional Diagnostics with Remote Loopback

Clear the Interface Statistics

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Ethernet Interfaces User Guide for Routing Devices

ON THIS PAGE

Verify Link and Transceivers with Pseudo Random Binary Sequence (PRBS) Test

How to Use Pseudo Random Binary Sequence (PRBS) Test for Link and Transceivers Verification

Configure Unidirectional Diagnostics

Configure Bidirectional Diagnostics with Remote Loopback

Clear the Interface Statistics

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