Running PRBS Test on QFX5240
What is PRBS
PRBS (Pseudo-Random Bit Sequence) is a deterministic test pattern used to validate the physical layer (PHY) of high-speed links such as Ethernet ports, SerDes lanes, optics, and cables.
PRBS operates below the Ethernet and packet layers and focuses purely on raw bit transmission quality.
Why PRBS Is Used
PRBS testing helps answer a fundamental question: Can this physical link reliably transmit bits at line rate?
It is commonly used to:
- Validate signal integrity on high-speed links (400G / 800G)
- Detect marginal optics, DAC/AEC cables, or connectors
- Identify lane-level issues in SerDes
- Expose clocking, jitter, or power-related instability
- Perform hardware bring-up and post-RMA validation
Common PRBS Patterns
| Pattern | Description | Typical Use |
|---|---|---|
| PRBS7 | Short pattern | Basic sanity tests |
| PRBS15 | Medium pattern | Extended stress testing |
| PRBS31 | Long pattern | Industry standard for 400G/800G |
PRBS31 is the most stressful pattern and is recommended for high-speed validation.
How PRBS Works
- Transmitter (Generator) generates a known PRBS pattern and sends raw bits.
- Receiver (Checker) regenerates the same pattern and compares incoming bits.
- Any mismatch is counted as a bit error.
PRBS on QFX5240
When PRBS is enabled on QFX5240:
- Normal Ethernet traffic is stopped
- The physical link goes Down (expected behavior)
- Interfaces enter PHY test mode
- Error counters are tracked per SerDes lane
Typical output:
PRBS Mode : Enabled
PRBS Pattern : 31
Lane X : Error Count : N
Interpreting Results
- Zero errors over time → Healthy link
- Slowly increasing errors → Marginal link
- Rapid error increase or failures → Faulty cable, optic, or lane
Lane-specific errors usually indicate:
- Defective SerDes lane
- Bad connector or cable
- Optical power or signal quality issue
PRBS vs Real Traffic
| Aspect | PRBS | Normal Traffic |
|---|---|---|
| Layer tested | PHY only | PHY + MAC + Network |
| Error sensitivity | Very high | Errors may be masked |
| Traffic | None | Packet-based |
| Use case | Hardware validation | Functional testing |
A link may pass traffic but fail PRBS, indicating marginal signal integrity.
I. Command Reference
Start / Stop PRBS Test
test chassis prbs fpc <fpc-slot> pic-slot <pic-slot> port <port-number | port-range> \
[channel <channel-number>] pattern <31> direction <tx|rx>-<start|stop>
Clear PRBS Statistics
clear interfaces statistics <interface-name>
Verify PRBS Status
show interfaces <interface-name>
II. Examples
A. PRBS Test on Non-Channelized 1x800G Ports
1. Check Initial State
show interfaces et-0/0/10
show interfaces et-0/0/11
2. Enable PRBS
test chassis prbs fpc 0 pic-slot 0 port 11 pattern 31 direction rx-start
test chassis prbs fpc 0 pic-slot 0 port 10 pattern 31 direction tx-start
3. Verify PRBS Statistics
show interfaces et-0/0/11
4. Stop PRBS
test chassis prbs fpc 0 pic-slot 0 port 10 pattern 31 direction tx-stop
test chassis prbs fpc 0 pic-slot 0 port 11 pattern 31 direction rx-stop
B. PRBS Test on Channelized 2x400G Ports
1. Configure Channelization
set interfaces et-0/0/20 number-of-sub-ports 2
set interfaces et-0/0/20 speed 400g
set interfaces et-0/0/24 number-of-sub-ports 2
set interfaces et-0/0/24 speed 400g
commit
2. Enable PRBS
test chassis prbs fpc 0 pic-slot 0 port 24 channel 0 pattern 31 direction rx-start
test chassis prbs fpc 0 pic-slot 0 port 24 channel 1 pattern 31 direction rx-start
test chassis prbs fpc 0 pic-slot 0 port 20 channel 0 pattern 31 direction tx-start
test chassis prbs fpc 0 pic-slot 0 port 20 channel 1 pattern 31 direction tx-start
3. Stop PRBS
test chassis prbs fpc 0 pic-slot 0 port 24 channel 0 pattern 31 direction rx-stop
test chassis prbs fpc 0 pic-slot 0 port 24 channel 1 pattern 31 direction rx-stop
test chassis prbs fpc 0 pic-slot 0 port 20 channel 0 pattern 31 direction tx-stop
test chassis prbs fpc 0 pic-slot 0 port 20 channel 1 pattern 31 direction tx-stop