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Field guide

Installation & handling

Physical handling of optical modules and fiber connectors. Most 'the link won't come up' tickets trace back to a problem covered in this document.

Next: Troubleshooting

Before you start

You'll need:

  • The module (in its anti-static bag)
  • The fiber patch cable (or copper, for RJ45 modules)
  • Fiber inspection scope (recommended for any new install)
  • One-click fiber cleaner or click-cleaner pen (LC, MPO, or both)
  • ESD wrist strap (recommended; required in production data centers)

ESD precautions

Optical modules contain laser diodes and high-speed CMOS circuitry, both sensitive to electrostatic discharge. A single 100 V ESD event, well below what you'd feel as a static shock, can damage the EEPROM, the laser, or both.

Before handling any module

  • Touch a grounded metal surface (the switch chassis ground lug is ideal)
  • Wear an ESD wrist strap clipped to a grounded point
  • Keep the module in its anti-static bag until you're at the port
  • Handle the module by its bail latch or its metal housing, never touch the gold electrical contacts
Dropped modules go in the recycle bin, not back in the box. A drop from desk height onto a hard floor can crack the laser package; the module may still link up but degrade rapidly.

Laser safety

Optical modules contain Class 1 laser products. Class 1 is "safe under all reasonably foreseeable conditions", but reasonable foresight means never looking directly into an active fiber or transceiver port. Some modules (ER, ZR, DWDM) operate at higher power and can damage the retina under prolonged direct exposure.

Always

  • Cap unused fiber connectors with the supplied bore caps
  • Cap unused transceivers with the supplied dust plug
  • Disable the port (shutdown on Cisco, disable on Junos, etc.) before disconnecting fiber if you'll be working at the open connector
  • Treat every fiber connector as if it might be live, even if you "know" the port is shut

Cleaning fiber connectors

The #1 cause of new-link bring-up failures is dirty fiber connectors. A single fingerprint on the end face attenuates the signal by 1–3 dB and can scatter enough light to corrupt the receive eye.

Every connector, every time

  1. Inspect the connector with a fiber scope at 200× or 400× magnification.
  2. If you see contamination, dust, oil, scratches, clean with a one-click cleaner. One click per connector pair.
  3. Re-inspect.
  4. Insert into the module.
Do not clean a connector with isopropyl alcohol and lint-free wipe unless you're a trained fiber technician. It's easy to make the contamination worse if you don't have the right technique. One-click cleaners are foolproof.
MPO connectors have 8, 12, or 16 ferrules in one body. All of them must be clean. Use an MPO-specific click cleaner, don't try to clean an MPO with an LC tool.

Installing the module

SFP / SFP+ / SFP28

  1. Verify the bail latch is closed (perpendicular to the module body).
  2. Orient the module: the bail latch is on the bottom for most chassis, top for others, check the switch chassis labeling.
  3. Slide the module fully into the cage until you feel it seat with a positive click. The bail should click into place.
  4. Connect the fiber. LC duplex connectors are keyed, there's only one way they fit. Match Tx to Rx (the connector body is usually marked).

QSFP+ / QSFP28 / QSFP56 / QSFP-DD / OSFP

  1. The bail latch on QSFP modules is on the side (a pull-tab).
  2. Confirm orientation: the bail tab faces the same direction across the entire port row on the chassis.
  3. Insert fully. QSFPs require firm pressure, about 4–5 lbs of insertion force. Don't be timid; modules that aren't fully seated cause intermittent link drops.
  4. For breakout cables, connect the MPO connector to the QSFP and each fanout LC to its respective host port.

OSFP power draw

OSFP modules (especially 800G and coherent ZR variants) can draw 15–20 W each, roughly double a QSFP28. Verify the switch chassis has adequate power and thermal headroom before populating every port.

RJ45 (copper SFP)

  1. Insert the module the same as a fiber SFP.
  2. Wait for the module's PHY to come up, copper SFPs take 5–15 seconds after insertion to enable, vs. <1 second for fiber. Don't worry if there's a brief delay before the link state shows up.
  3. Copper SFPs run hotter than fiber SFPs. In dense deployments (48 ports of copper), make sure airflow is adequate.

Cabling polarity

For QSFP+ SR4 and QSFP28 SR4, the MPO trunk cable's polarity must match the module's expected polarity.

PolarityDescriptionCommon use
Type AStraight-throughSome legacy fabrics
Type BReversedCurrent standard for data centers
Type CPair-flipped (per pair)Specialized, rare

For new installs

Standardize on Type B for everything. It's the dominant polarity in modern data-center fabric.

Symptoms of polarity mismatch

The link doesn't come up at all, or comes up as half-duplex / negotiation failure. The fix is either a polarity adapter at one end or replacing the trunk cable with the correct polarity.

Bringing up the link

Cisco IOS-XE / NX-OS

Switch# configure terminal
Switch(config)# interface gigabitEthernet 1/0/49
Switch(config-if)# no shutdown
Switch(config-if)# end
Switch# show interface gigabitEthernet 1/0/49 transceiver detail
Switch# show interface gigabitEthernet 1/0/49 status

Expect to see connected in the status output and Vendor Name: CISCO (or your custom name) in the transceiver detail.

Juniper Junos

> configure
# set interfaces et-0/0/0 enable
# commit
> show interfaces et-0/0/0 extensive
> show interfaces diagnostics optics et-0/0/0

Arista EOS

arista(config)# interface ethernet 1
arista(config-if-Et1)# no shutdown
arista# show interfaces ethernet 1 transceiver detail
arista# show interfaces ethernet 1 status

Verifying optical health

Within 30 seconds of link-up, capture baseline DDM values. These are what "normal" looks like for this specific module on this specific fiber. Future degradation is measured against this baseline.

Capture and store

  • Tx optical power (dBm)
  • Rx optical power (dBm)
  • Laser bias current (mA)
  • Module temperature (°C)

Removing a module

  1. Disable the port (shutdown / disable) before unplugging fiber.
  2. Disconnect the fiber. Cap the connectors immediately.
  3. Open the bail latch (pull down for SFP, pull sideways for QSFP/OSFP). Pull the module out using the latch as a handle, never yank by the fiber.
  4. Cap the module's bore with the supplied dust plug.
  5. Store in an anti-static bag.
Hot-swap is supported on every module we sell, but it's still good practice to shut the port first. Hot-removing a module under traffic can occasionally trigger ASIC errors on older platforms.