Cisco Router Fiber Optic Levels

When troubleshooting a fiber link on a Cisco router, one of the first things I check is the transmit and receive optical power. These values quickly tell you whether the optic is receiving usable light, whether the signal is getting weak, or whether there’s a problem somewhere in the optical path.

The important part is understanding that there is no single good Tx or Rx value for every optic. The acceptable range depends on the exact transceiver installed. A light level that is bad for one optic can be completely normal for another.

Architect’s Note: Don’t memorize one universal dBm range and apply it to every fiber link. Identify the optic first, then compare the live Digital Optical Monitoring values against the thresholds for that specific transceiver.

What Tx and Rx Optical Power Mean
What Is a Good Optical Power Level?
Example Optical Ranges
Find the Interface on the Router
Check the Optical Levels from the CLI
Understanding Warning and Alarm Thresholds
100G C8500 Example
Why 100G Optics Show Multiple Lanes
What Weak Light Usually Looks Like
Troubleshooting Workflow
Useful Cisco Commands
Final Takeaway

01What Tx and Rx Optical Power Mean

Tx power is the amount of optical power the local transceiver is transmitting into the fiber. Rx power is the amount of optical power the local transceiver is receiving from the device on the other end of the fiber.

Both values are normally displayed in dBm. Because dBm is logarithmic, a difference of 10 dB represents a tenfold difference in optical power. This is one reason optical ranges can look large when you first look at them.

Why It Matters: Rx power is usually the first value I focus on when troubleshooting the physical fiber path. If the remote device is transmitting normally but the local Rx value is weak, the problem may be somewhere between the remote optic and the local receiver.

02What Is a Good Optical Power Level?

There is no universal answer. A 10G SR optic, a 10G LR optic, and a 100G LR4 optic all have different transmit and receive specifications. The correct way to evaluate an optical reading is to compare the measured value against the supported range or the Digital Optical Monitoring thresholds reported for the installed transceiver.

For example, a receive level around -14 dBm would be well outside the normal operating range of some short-reach optics. That same reading can be within specification for an optic designed for a much longer optical path. This is why using one hard value such as -14 dBm as a universal failure threshold is not accurate.

Common Mistake: Don’t assume that a more negative number automatically means the fiber is bad. The reading must be compared against the limits of the exact optic installed in the router.

03Example Optical Ranges

These are examples only. Always verify the installed optic and its reported thresholds.

Common Optic	Approx. Tx Range	Approx. Rx Range	Typical Reach
10G SR	-7.3 to -1.2 dBm	-9.9 to -1.0 dBm	Short reach multimode
10G LR	-8.2 to +0.5 dBm	-14.4 to +0.5 dBm	Up to 10 km single mode
10G ER	-4.7 to +4.0 dBm	-15.8 to -1.0 dBm	Up to 40 km single mode
100G LR4 (QSFP-100G-LR4-S)	-4.3 to +4.5 dBm per lane	-10.6 to +4.5 dBm per lane	Up to 10 km single mode

Cisco specifies the QSFP-100G-LR4-S with four optical lanes, a per-lane Tx range of -4.3 to +4.5 dBm, and a per-lane Rx range of -10.6 to +4.5 dBm.

04Find the Interface on the Router

On older ASR platforms you may be checking a 10G TenGigabitEthernet interface. On a Catalyst 8500 using 100G optics you’ll commonly be working with HundredGigE interfaces. For a C8500, I usually start by identifying the HundredGigE interfaces:

EDGE-RTR01#show ip interface brief | include Hu
Hu0/1/0                unassigned      YES unset  up                    up
Hu0/1/1                unassigned      YES unset  up                    up
Hu0/1/2                unassigned      YES unset  up                    up
Hu0/1/3                unassigned      YES unset  up                    up

The interface name maps directly to the hardware location. For example, Hu0/1/0 is slot 0, subslot 1, port 0.

05Check the Optical Levels from the CLI

The command I use most often is:

show hw-module subslot 0/1 transceiver 0 status

For another port, change the transceiver number:

show hw-module subslot 0/1 transceiver 1 status

You can also query the interface directly on supported IOS XE platforms:

show interfaces HundredGigE0/1/0 transceiver detail

To identify the exact transceiver installed, use the IDPROM information:

EDGE-RTR01#show hw-module subslot 0/1 transceiver 0 idprom detail
IDPROM for transceiver HundredGigE0/1/0:
  Description                               = QSFP28 optics
  Transceiver Type:                         = QSFP 100GE LR4
  Product Identifier (PID)                  = QSFP-100G-LR4-S
  Vendor Revision                           = 01
  Serial Number (SN)                        = XXXXXXXXXXX
  Device State                              = Enabled

Architect’s Note: The IDPROM check is important. Before deciding that an optical value is good or bad, verify whether the port contains an SR, LR, ER, LR4, or another optic type.

06Understanding Warning and Alarm Thresholds

A detailed transceiver output may show four thresholds for optical power: High Alarm, High Warning, Low Warning, and Low Alarm. A sanitized 100G LR4 example may look like this:

Optical Receive Power

Port       Lane   Receive     High Alarm   High Warn   Low Warn    Low Alarm
                    dBm          dBm          dBm         dBm         dBm
---------  -----  ----------   ----------   ---------   ---------   ---------
Hu0/1/0      1      -1.38         7.50         4.50       -10.61      -14.61
Hu0/1/0      2      -1.77         7.50         4.50       -10.61      -14.61
Hu0/1/0      3      -0.98         7.50         4.50       -10.61      -14.61
Hu0/1/0      4      -1.36         7.50         4.50       -10.61      -14.61

These receive values are healthy. Every lane is comfortably inside the normal operating range for the optic.

Reading	Interpretation for This LR4 Example
Rx around -1 to -3 dBm	Strong received signal, comfortably within specification
Rx around -7 dBm	Still within specification
Rx around -10 dBm	Still near the supported lower limit, but optical margin is getting small
Rx below approx. -10.6 dBm	Below the normal receive specification and into the low warning area
Rx below approx. -14.6 dBm	Below the example module’s low alarm threshold
Rx around -30 to -40 dBm	Usually little or no usable received light

Important: The warning and alarm numbers above are from this example optic. Don’t apply them to a different transceiver without checking its own thresholds.

07100G C8500 Example

Here is an example from a Catalyst 8500 using a 100G LR4 optic. The router name and serial information have been sanitized.

EDGE-RTR01#show hw-module subslot 0/1 transceiver 0 status
The Transceiver in slot 0 subslot 1 port 0 is enabled.
  Module temperature                        = +43.562 C
  Transceiver Tx supply voltage             = 3.2974 Volts
  Transceiver Tx power                      = 7.5 dBm
  Transceiver Rx optical power              = 4.6 dBm

  Tx power Network Lane[00]                 = 1.7 dBm
  Tx power Network Lane[01]                 = 1.4 dBm
  Tx power Network Lane[02]                 = 1.3 dBm
  Tx power Network Lane[03]                 = 1.6 dBm

  Rx power Network Lane[00]                 = -1.4 dBm
  Rx power Network Lane[01]                 = -1.8 dBm
  Rx power Network Lane[02]                 = -1.0 dBm
  Rx power Network Lane[03]                 = -1.4 dBm

The individual lane values are what I care about here. Tx is approximately +1.3 to +1.7 dBm per lane. Rx is approximately -1.0 to -1.8 dBm per lane. Those values are comfortably inside the supported range of a Cisco 100G LR4 optic.

Another healthy port could look like this:

EDGE-RTR01#show hw-module subslot 0/1 transceiver 1 status
The Transceiver in slot 0 subslot 1 port 1 is enabled.
  Module temperature                        = +41.093 C
  Transceiver Tx supply voltage             = 3.2901 Volts

  Tx power Network Lane[00]                 = 2.4 dBm
  Tx power Network Lane[01]                 = 2.6 dBm
  Tx power Network Lane[02]                 = 1.8 dBm
  Tx power Network Lane[03]                 = 2.2 dBm

  Rx power Network Lane[00]                 = -3.3 dBm
  Rx power Network Lane[01]                 = -3.1 dBm
  Rx power Network Lane[02]                 = -3.2 dBm
  Rx power Network Lane[03]                 = -2.6 dBm

Those values are also normal. A receive value of -3 dBm is not worse simply because it is more negative than -1 dBm. Both are well inside the supported operating range of this optic.

08Why 100G Optics Show Multiple Lanes

A 100G LR4 optic uses four optical wavelengths. Each lane carries part of the total 100G signal, so IOS XE can show Tx power and Rx power for each individual lane.

Tx power Network Lane[00]
Tx power Network Lane[01]
Tx power Network Lane[02]
Tx power Network Lane[03]

Rx power Network Lane[00]
Rx power Network Lane[01]
Rx power Network Lane[02]
Rx power Network Lane[03]

For a multi-lane optic, evaluate the individual lane readings. You want all of the lanes to remain inside the supported range, and you also want to look for a lane that is significantly different from the others.

Real World Example: If three Rx lanes are around -3 dBm and one lane is near -10 dBm, I would investigate that optic or optical path even if the interface is still up. The imbalance is useful troubleshooting data.

Cisco has also documented an IOS XE issue, CSCwa52627, where the summarized Tx or Rx value for some QSFP transceivers can be incorrect. When the summary value does not agree with the individual lane readings, use the lane readings for the actual optical levels and verify whether your IOS XE release includes the fix.

Common Mistake: Don’t add dBm lane values together. dBm is logarithmic. If you ever need total optical power, convert each lane to milliwatts, add the milliwatt values, then convert the total back to dBm.

09What Weak Light Usually Looks Like

When receive power begins approaching the low warning threshold, there is less optical margin. The link may still be up, but this is where I start looking more closely at the physical path.

Extremely low readings, often around -30 to -40 dBm depending on the optic and platform, usually mean the receiver is seeing little or no usable optical signal. Possible causes include:

Fiber is not connected
Tx and Rx polarity is reversed
Remote optic is not transmitting
Dirty or contaminated connector
Connector is not fully seated
Damaged patch cable
Excessive connector, splice, or patch panel loss
Incorrect optic or fiber type
Fault somewhere in a carrier or cross-connect path

Don’t assume that every low reading means the local patch cable is bad. The entire optical path matters.

10Troubleshooting Workflow

This is the workflow I recommend when checking light levels on a Cisco router.

Identify the physical interface.
Identify the exact optic installed.
Check Tx and Rx optical power.
For 100G QSFP optics, check every individual lane.
Compare the readings against that optic’s supported range and DOM thresholds.
Check both ends of the link. The local Rx value is produced by the remote Tx side.
Look for lane imbalance. One abnormal lane can be important even when the interface remains up.
Investigate the physical path if levels are near or outside warning thresholds.

Why It Matters: Looking at both ends gives you direction. If Router A has weak Rx, check Router B’s Tx and everything in the optical path from Router B toward Router A. This helps narrow the fault domain instead of replacing optics and patch cables at random.

11Useful Cisco Commands

Find 100G interfaces:

show ip interface brief | include Hu

Check the transceiver status:

show hw-module subslot 0/1 transceiver 0 status

Display detailed thresholds using the interface form where supported:

show interfaces HundredGigE0/1/0 transceiver detail

Identify the optic:

show hw-module subslot 0/1 transceiver 0 idprom detail

Check the router inventory:

EDGE-RTR01#show inventory
NAME: "Chassis", DESCR: "Cisco C8500-20X6C Chassis"
PID: C8500-20X6C, VID: V01, SN: XXXXXXXXXXX

NAME: "subslot 0/1 transceiver 0", DESCR: "QSFP 100GE LR"
PID: QSFP-100G-LR4-S, VID: 01, SN: XXXXXXXXXXX

12Final Takeaway

Checking optical power on a Cisco router is straightforward. The part that matters is interpreting the values correctly.

Don’t start with the assumption that -3 dBm is good, -10 dBm is questionable, and -14 dBm is bad. Start by identifying the transceiver. Then compare the actual Tx and Rx readings against the specifications and DOM thresholds for that optic.

On modern 100G Catalyst 8500 interfaces, pay particular attention to the individual optical lanes. Those lane values give you the most useful view of what the optic is actually transmitting and receiving.

Quick Reference: Identify the optic, check the individual Tx and Rx values, compare them to that optic’s thresholds, check both ends of the link, and investigate the optical path when readings approach or cross the warning thresholds.

Cisco Router Fiber Optic Levels

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