Connectrix: How to Interpret SFP Transceiver TX and RX Power Levels as Part of Troubleshooting the SFP Transceiver
Summary: Using the measured light power levels displayed in the sfpshow (Brocade) and the show interface transceiver details (Cisco) to identify physical layer issues with switch to switch (ISL) and node to switch connections. ...
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Symptoms
- Adding ISL or trunk connections
- Expanding distance on existing ISL connections
- End device HBA or SFP replacement or upgrade
- Customer infrastructure (cabling, patch panel, DCM, CWDM) issues or changes
- Replacing old hardware with new hardware and using the same physical infrastructure (Cables, patch panels.)
Physical layer issues on connections can lead to connection instability causing:
- End devices to log in and out of the fabric
- Can cause impact to how the link performs.
- Stops End device logging in
- Causes issues with fabric expansion
- SFP TX and RX power readings are not correctly interpreted.
- SFP is incorrectly replaced based on false interpretations.
- Interpret show interface fcx/xx transceiver details TX and RX Power.
- Interpret sfpshow 8/22 f TX and RX Power.
Cause
- Wrong interpretation of the TX or RX counters of the SFP outputs causes unnecessary SFP replacements.
- Incorrect switch port SFPs with the cable length.
- Incorrect cable
- Customer infrastructure (cabling, patch panel, DCM, CWDM) issues or changes.
- Failing switch port SFP.
Resolution
This article is intended to assist with the interpretation of the SFP transceiver TX and RX power readings available from the CLI.
It must also be used as an aid to the following articles:
As can be seen the SFP has two receptacles into which the FC fibre cable is connected, each receptacle on the SFP represents its own channel, one channel for sending TX and the other for receiving RX making the SFP module a device that functions as both at transmitter and a receiver that is combined in the same housing that is otherwise known as a transceiver.
Logically the FC connection looks as follows:
In the example above, you could change the SFP with an HBA and it follows the same structure but for explanation we use an ISL connection. On the left is a switch port SFP which is connected using a cable to the right of another switch port SFP.
The red channel left TX transceiver is the source of the light signal. Looking at the TX power on this SFP, you are viewing the source. Looking at the power level on the right SFP RX power, you are viewing the light energy that is created by the left SFP as it passes through the cable and reaches the right SFP and the opposite way for the green channel.
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Cisco CLI command
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As shown in red at the end of both command outputs are the TX and RX power readings, looking from left to right, the first value = the measured value the next two values are a high low limit where if the measured the value exceeds, the switch sends an alarm and the next two values are high low limits that are considered to be warning.
Because of this, there are wide but clearly defined set of limits that all conform to IEEE standards BER (bit error ratio).
If RX and TX power readings are of any concern and Rx power is lower than expected, we would first look to the above factors and apply the Pre SFP replacement procedure (see above) with the additional step of collecting the SFPshow -f (Brocade) or show interface transceiver details (Cisco) command outputs as follows:
It must also be used as an aid to the following articles:
- Pre SFP Replacement Procedure
- Connectrix: How to troubleshoot Fibre Channel node to switch port or SFP communication problems by elimination. See Dell KB article 28863: Connectrix: How to troubleshoot Fibre Channel node to switch port or SFP communication problems by means of elimination?
As can be seen the SFP has two receptacles into which the FC fibre cable is connected, each receptacle on the SFP represents its own channel, one channel for sending TX and the other for receiving RX making the SFP module a device that functions as both at transmitter and a receiver that is combined in the same housing that is otherwise known as a transceiver.
Logically the FC connection looks as follows:
In the example above, you could change the SFP with an HBA and it follows the same structure but for explanation we use an ISL connection. On the left is a switch port SFP which is connected using a cable to the right of another switch port SFP.
The red channel left TX transceiver is the source of the light signal. Looking at the TX power on this SFP, you are viewing the source. Looking at the power level on the right SFP RX power, you are viewing the light energy that is created by the left SFP as it passes through the cable and reaches the right SFP and the opposite way for the green channel.
How to view the light levels
Brocade CLI command:
-
Sfpshow [slot]/port f admin> sfpshow 8/22 -f Identifier: 3 SFP
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Alarm Warn low high low high Temperature: 46 Centigrade -5 85 0 75 Current: 7.428 mAmps 2.500 12.000 2.000 11.500 Voltage: 3295.6 mVolts 3000.0 3600.0 3130.0 3460.0 RX Power: -3.3 dBm (466.6 uW) 31.6 uW 1258.9 uW 31.6 uW 794.0 uW TX Power: -2.5 dBm (560.4 uW) 126.0 uW 1258.9 uW 251.0 uW 794.0 uW
Cisco CLI command
- Show interface fc module/port transceiver details.
# show interface fc1/12 transceiver details
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---------------------------------------------------------------------------- Alarms Warnings High Low High Low ---------------------------------------------------------------------------- Temperature 47.05 C 89.00 C -9.00 C 85.00 C -5.00 C Voltage 3.30 V 3.60 V 3.00 V 3.50 V 3.10 V Current 5.47 mA 10.00 mA 2.00 mA 10.00 mA 2.00 mA Tx Power -4.87 dBm 1.00 dBm -13.50 dBm -3.00 dBm -9.50 dBm Rx Power -7.69 dBm 4.00 dBm -21.02 dBm 0.00 dBm -16.99 dBm Transmit Fault Count = 0 ---------------------------------------------------------------------------- Note: ++ high-alarm; + high-warning; -- low-alarm; - low-warning
Units of measurement
As seen in the brocade sfpshow output, the light level is represented in dBm and uW.
Decibels (dB) is the ratio of output power to input power, expressed as 10*log (power ratio).
dBm = power in decibels relative to 1mW (1000uW). A reading of 0 dBm is 1mW.
If the power reading is plus (+) dBm the power is >1mW, minus (-) is <1mW.
For example, in the Brocade sfpshow output above, 466uW is 10*log(466/1000) = -3.3dBm.
Also displayed is the light power in uW.
uW = micro Watts which is the actual measure of the light power (not a ratio)
Cisco only gives the reading in dBm.
Understand readings
Once the TX and RX power readings are within the limits that are provided in the command output the SFP is receiving and transmitting light within acceptable levels, because they receive power as measured on an SFP is subject to several external factors:
- Fibre infrastructure
- Patch panel health
- Cabling health
- Cabling length
- Number of splices causing losses due to refraction
- Temperature affects the light measurement.
Because of this, there are wide but clearly defined set of limits that all conform to IEEE standards BER (bit error ratio).
If RX and TX power readings are of any concern and Rx power is lower than expected, we would first look to the above factors and apply the Pre SFP replacement procedure (see above) with the additional step of collecting the SFPshow -f (Brocade) or show interface transceiver details (Cisco) command outputs as follows:
- Before the cable move
- After the cable move
- Move the cable back to the original port in step 1.
- If the RX power is low in step 1, is ok in step 2 and is low in step 3 using the same cabling, then it indicates an SFP issue where SFP cleaning should be applied (see Dell KB article 45672: Connectrix, Symmetrix, CLARiiON, Celerra, VNX, RecoverPoint, and VPLEX: Use cases for Fibre Channel inspection and cleaning kit deployment.) and the test carried out again after cleaning, if retest yields the same outcome replace the SFP.
- If the RX power is low in step 1, ok for step 2, ok for step 3 it would indicate a dirty cable connector or SFP and during the test the debris causing the original issue was dislodged or move this would still be a candidate for cable and or SFP cleaning and retest.
- If the RX power is low in step 1, low in step 2 and low in step 3 this suggests the issue external to the switch port SFP and indicates an issue with the cabling infrastructure or end device.
NOTE:
The number 1 issue related to link related problems are dirty cables and optics, many times it is incorrectly assumed to be a faulty optic.
Example would be when in a scenario where a new higher speed (16 Gb) switch is rolled into an existing environment and then old and existing hosts and targets are installed with existing fiber cables.
This is such a wide spread and common event that there is an optic cleaning kit available with the Dell field team to alleviate these issues. In other words, do not assume it is the optic. More information about the cleaning kit can be found in Dell KB article 45672: Connectrix, Symmetrix, CLARiiON, Celerra, VNX, RecoverPoint, and VPLEX: Use cases for Fibre Channel inspection and cleaning kit deployment.
Additional Information
Addition resources:
Primary Link to Brocade SFP data sheets:
http://www.brocade.com/products/all/transceivers/product-details/transceiver-modules/specifications.page
Primary link for Cisco SFP data sheets:
http://www.cisco.com/c/en/us/products/collateral/storage-networking/mds-9000-series-multilayer-switches/product_data_sheet09186a00801bc698.html
Affected Products
ConnectrixProducts
Connectrix, Connectrix B-Series Hardware, Connectrix MDS-Series HardwareArticle Properties
Article Number: 000028053
Article Type: Solution
Last Modified: 26 May 2022
Version: 5
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