Description

The Tx/Rx power windows of 40G QSFP+-40G-SR4 and 10G SR/USR do not line up. It is recommended to use channelized QFX-QSFP-40G-ESR4 for 4 x 10G break-out to be connected with 10G SFPs, not QFX-QSFP-40G-SR4.

Symptoms

Topology

QFX5200-1 xe-0/0/53:0(QSFP+-40G-SR4)-----------------( SFP+-10G-SR)xe-0/0/44 QFX5100-2
xe-0/0/53:1 ----------------------------------------------------- xe-0/0/45
xe-0/0/53:2 ----------------------------------------------------- xe-0/0/46
xe-0/0/53:3 ----------------------------------------------------- xe-0/0/47

When connecting channelized interfaces (using QSFP+-40G-SR4) to peer 10G interfaces(SFP+-10G-SR), we may see rx power high warning on the peer SFP+-10G-SR.
 
admin@qfx5100-48s-01# run show interfaces diagnostics optics xe-0/0/4[4-7] |match "receive|rx power high warning|phys"
Physical interface: xe-0/0/44
Receiver signal average optical power : 0.9413 mW / -0.26 dBm
Laser rx power high warning : On
Laser rx power high warning threshold : 0.7943 mW / -1.00 dBm
Physical interface: xe-0/0/45
Receiver signal average optical power : 0.8930 mW / -0.49 dBm
Laser rx power high warning : On
Laser rx power high warning threshold : 0.7943 mW / -1.00 dBm
Physical interface: xe-0/0/46
Receiver signal average optical power : 0.9650 mW / -0.15 dBm
Laser rx power high warning : On
Laser rx power high warning threshold : 0.7943 mW / -1.00 dBm
Physical interface: xe-0/0/47
Receiver signal average optical power : 0.8618 mW / -0.65 dBm
Laser rx power high warning : On
Laser rx power high warning threshold : 0.7943 mW / -1.00 dBm

The output power on the QSFP+-40G-SR4 interfaces are within normal rage:
 
admin@qfx5100-48t-00# run show interfaces diagnostics optics xe-0/0/48:0 |match "Laser output power :|Lane"
Lane 0
Laser output power : 0.958 mW / -0.19 dBm
Lane 1
Laser output power : 1.013 mW / 0.06 dBm
Lane 2
Laser output power : 1.025 mW / 0.11 dBm
Lane 3
Laser output power : 0.952 mW / -0.21 dBm

QFX-QSFP-40G-SR4 Laser output threshold is [-7.6 dBm ~ 2.4 dBm] based on Interface Specifications for SFP, SFP+, and QSFP+ Transceivers for the QFX Series

SFP+-10G-SR Laser rx power threshold is [-9.9 dBm ~ -1.0 dBm]

This means when output power of the QFX-QSFP-40G-SR4 is between -1.0 and 2.4 which is within normal range, it will exceed the peer SFP+-10G-SR’s normal range.

Solution

This is because the Tx/Rx power windows of 40G QSFP+-40G-SR4 and 10G SR/USR do not line up.

It is recommended to use channelized QFX-QSFP-40G-ESR4 for 4 x 10G break-out to be connected with 10G SFPs, not QFX-QSFP-40G-SR4.

Notes:

  1. When you want to channelize and use 40G ports, you need to use QFX-QSFP-40G-ESR4.
  2. When you don't need channelization, use QFX-QSFP-40G-SR4.

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