The Tool Desk
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What packet loss and latency mean on an optical link
An optical interface can remain administratively and physically up while discarding frames, correcting errors, or queuing traffic long enough to cause application-visible delay. “Link up” only proves that the interface has a carrier state; it does not prove that every received frame is valid or that queues are empty. Treat packet loss and latency as two observations that must be correlated with counters and events.
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Packet loss caused by corruption usually leaves evidence in receive or physical-layer counters. Packet loss caused by congestion usually appears in queues, pause behavior, RED or tail drops, and output-resource counters while physical-error counters stay stable. Latency can rise before packets are discarded, because buffers fill first. It can also spike when a link briefly flaps, when forward error correction (FEC) is working hard, or when one lane of a parallel optic is degraded.
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- Congestion or policy: queue depth, pause frames, RED/tail drops, output drops, or resource counters increase without a matching rise in physical errors.
- Intermittent link instability: carrier transitions, loss-of-signal alarms, remote-fault messages, or repeated flaps line up with the loss or latency event.
The practical objective is not to guess which box is bad. It is to prove whether the symptom follows the cable, optic, port, lane, or traffic condition.
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1. Establish a useful baseline before changing anything
Write down the complete path and its operating parameters before clearing counters or reseating parts. Record the local and remote interface identifiers, optic part numbers and vendor coding, negotiated speed, fiber mode, connector type, breakout arrangement, MTU, and FEC mode. Note which traffic classes are affected and the exact timestamps of packet loss or latency spikes.
Capture the current counters on both ends, then repeat the capture after a defined observation window. Deltas are more informative than lifetime totals: an old CRC count may describe a problem that was fixed months ago, while a rapidly increasing count identifies an active fault. Use the same interval and traffic conditions for every comparison. Preserve a copy of the initial readings so that a later “clear counters” operation does not erase your evidence.
Measure latency in a way that distinguishes the link from the endpoints. Compare a continuously monitored flow with interface timestamps, queue statistics, and application reports. If only one class of service spikes, examine its queue and policy counters rather than assuming the optical module is failing.
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Review event logs for the local and remote interfaces at the same time range as the loss. Look for carrier transitions, loss-of-signal or loss-of-lock alarms, remote-fault indications, optic-insertion events, and negotiation or compatibility messages. A flap recorded on one end but not the other can indicate which receive path first lost a valid signal.
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Cisco’s port-flap workflow is a useful model: confirm the event in logs, validate physical connectivity and optics, review counters, and use digital optical monitoring (DOM) when the transceiver supports it. Do not infer that a quiet log means a healthy link; a degraded signal can produce corrected or discarded frames without a carrier transition.
Correlate time, direction, and interface
- Align the timestamp of a latency spike with counter deltas, not just with a once-per-day summary.
- Check both directions. A transmitter can look normal while the receiver on the far end reports CRC or FEC errors.
- Compare the physical interface with the logical port-channel or breakout member. One bad member can create loss that appears to be an aggregate-link problem.
3. Read error counters as a pattern
Cisco defines a CRC error as a checksum mismatch and an FCS error as a frame with a bad frame-check sequence; frames with FCS errors are dropped. Cisco lists noise or transmission problems, bad cabling, a bad port or NIC, damaged optics, and duplex mismatch among common causes. On modern full-duplex data-center links, a duplex mismatch is less common than a damaged or incompatible physical component, but it remains a valid check on platforms that support the setting.
Juniper’s interface guidance separates physical-layer indications such as PCS drops and uncorrected FEC errors from Ethernet and resource counters. PCS drops indicate possible optical signal degradation. Uncorrected FEC errors identify errors that FEC could not repair when FEC is enabled. Use these distinctions to narrow the fault domain.
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| Observed pattern | Most likely area | Next check |
|---|---|---|
| CRC, FCS, symbol, or framing errors increase in one direction | Transmitter-to-receiver path: optic, patch lead, patch panel, remote port, or NIC | Inspect DOM at both ends, clean and reseat according to site procedure, then swap one cable or optic |
| PCS drops or uncorrected FEC errors increase | Optical signal degradation, lane fault, unsupported optic, polarity issue, or failing fiber | Compare per-lane telemetry and FEC mode; verify fiber and optic compatibility |
| Pause frames, queue depth, RED/tail drops, output drops, or resource errors increase while physical counters stay flat | Congestion, oversubscription, buffering, class-of-service, or policy | Trace the affected queue, traffic class, and egress capacity |
| Carrier transitions or repeated flaps | Seating, optic alarms, polarity, port hardware, or compatibility | Correlate logs on both ends and test with known-good components |
| Oversized-frame or input/output errors without a matching optical indication | MTU, framing, interface configuration, or platform resource issue | Compare MTU and encapsulation on both sides and inspect platform-specific resource counters |
A single counter is not a diagnosis. For example, a large lifetime CRC total may be harmless if it stopped increasing, while a small but accelerating uncorrected-FEC count deserves immediate attention.
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4. Separate corruption from congestion
Signs of a physical or optical fault
Corruption is supported by increasing CRC/FCS, symbol, framing, PCS, or uncorrected-FEC counters, especially when the increase is directional and coincides with DOM alarms or link events. Applications may report retransmissions and latency variance even when the interface never goes down. FEC can hide some bit errors by correcting them, so corrected-error growth is an early warning; uncorrected errors show that the correction budget has been exceeded.
Signs of congestion or policy pressure
When physical counters remain clean but queue depth, pause frames, RED or tail drops, output drops, or resource-exhaustion counters rise, investigate traffic demand and scheduling. Check whether multiple ingress links oversubscribe one egress, whether a class-of-service queue is undersized, and whether shaping, policing, or a microburst is responsible. Latency in this case is queueing delay, not propagation delay. Replacing an optic will not increase egress capacity or remove a queue policy.
When both appear together
Congestion and corruption can coexist. A bad link may trigger retransmissions that increase offered load, while a congested queue can obscure the timing of physical errors. Establish which counter began rising first and compare unaffected links carrying similar traffic. Keep the two hypotheses separate until a controlled test changes one variable.
5. Use DOM and lane telemetry as evidence
On supported transceivers, inspect receive optical power, transmit power, temperature, supply voltage, TEC current, laser bias, and per-lane values. Juniper’s DOM output also exposes alarms such as receive loss of signal and clock-data-recovery (CDR) loss of lock. Capture readings at both ends at the time of the symptom and again during a quiet period.
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Do not use a single vendor threshold as a pass/fail verdict. A value inside its stated range can still be unstable, asymmetric, or drifting toward an alarm. Compare the near-end transmit value with the far-end receive value, and compare lanes within the same module. A weak receive lane, a rapidly changing bias current, or a temperature excursion is more informative when it changes in step with FEC or CRC deltas.
- Receive power: low or fluctuating readings support loss, contamination, bend, connector, or transmitter-path problems; an unexpectedly high reading can also indicate an unsuitable reach or optic combination.
- Transmit power and laser bias: changes can indicate a stressed or aging transmitter, but interpret them with the module’s specifications and temperature.
- Temperature and voltage: excursions can produce instability or alarms even when optical power looks normal.
- Per-lane values: one abnormal lane on a parallel optic points toward lane mapping, breakout, connector, or a single-lane failure.
6. Verify fiber, optic, and FEC compatibility
Juniper’s compatibility guidance is explicit: connect single-mode fiber only to single-mode interfaces, multimode fiber only to multimode interfaces, and use the correct SFP at both ends. Confirm the installed fiber type, connector polish and form, supported reach, and the optic’s rated speed before swapping hardware.
For 40G, 100G, and faster parallel links, verify polarity, lane mapping, and breakout mode. A connector can be seated while lanes are crossed or mapped for a different breakout, producing persistent PCS or FEC symptoms. Check that both ends support the same speed and that the platform accepts the optic’s vendor coding.
FEC is not interchangeable across every optic and speed. Cisco notes that FEC is enabled or selected according to the optical module and that interface capabilities expose FEC support. Confirm the required mode at both ends instead of forcing a setting copied from another link. A mismatch can prevent the link from coming up or leave it operating with excessive uncorrected errors.
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|---|---|---|
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| Reach and wavelength | Optic rating, wavelength, and installed distance | A module designed for a different distance can overload or under-drive the receiver |
| Connector and polarity | Connector type, keying, polarity, and patch-panel orientation | Incorrect polarity or contamination can affect one or more lanes |
| Speed and breakout | Port speed, lane count, breakout profile, and mapping | Parallel lanes may not align with the configured logical interfaces |
| FEC | Supported and selected mode on both interfaces | Incompatible FEC can cause link failure or uncorrected errors |
| Vendor coding | Platform support for the optic identifier and firmware | An unsupported module may report alarms or behave unpredictably |
7. Isolate one component at a time
Controlled substitution is more reliable than replacing several parts at once. Before each change, record and, where operationally safe, clear the relevant counters. Maintain the same speed, fiber mode, FEC, and traffic so that the result is comparable.
- Inspect and reseat: Follow the site’s connector-cleaning procedure, check latches and dust caps, and reseat the patch lead and optic. Never look into an active fiber connector.
- Replace the patch lead: Use a known-good lead of the same fiber mode, connector type, polarity, and suitable reach. Observe both-end counters for the same fixed interval.
- Replace one optic: Substitute a known-good, supported optic of the same speed, reach, coding, and FEC requirements. Change only one end first so that the direction of improvement is clear.
- Move to a known-good port: Keep the cable and optic unchanged, then move the link to a port with a documented clean history and compatible capabilities. Follow change-control and port-channel procedures to avoid creating a second outage.
- Test the remote device: If errors remain with a new cable, optic, and port, inspect the remote port, line card, ASIC, or NIC. A component that makes the errors follow it is the leading suspect; errors that stay with one port implicate that port or its underlying hardware.
For a port-channel, test the individual member that shows the counter growth rather than replacing the entire bundle. Keep an explicit record of which swap changed the error rate, DOM readings, and latency; otherwise a later rollback can erase the causal clue.
8. Test the external fiber path when switch evidence points outside
If both ends show optical alarms or persistent physical errors after compatible optics and patch leads have been tested, examine the installed fiber path. Juniper recommends an external cable diagnostic testing tool for fiber integrity. Select an instrument that matches the fiber type, connector, reach, and the acceptance standard required by your site.
| Tool type | Best use | Limitation |
|---|---|---|
| Fiber-optic cable tester | General integrity checks on an installed link or patch path | Capabilities vary; verify support for the cable and connectors in use |
| Optical power meter | Measure received or transmitted optical power against a reference | Does not by itself locate every event or certify all link parameters |
| Visual fault locator | Find obvious breaks, severe bends, or incorrect continuity over short paths | Limited reach and diagnostic depth; follow laser-safety procedures |
| Certification tester | Acceptance testing against the site’s cabling standard | Requires the correct modules, test cords, and trained operator |
Disconnect the link or follow the facility’s approved maintenance method before inserting test equipment. Record launch conditions, reference method, connector cleaning, and test results so that a contractor or cabling team can reproduce the finding. A switch DOM reading can suggest a bad path; an external test supplies the independent evidence needed for repair or acceptance.
9. Close the loop with monitoring and trend data
After remediation, continue collecting counters, latency, DOM, BER, FEC, and link-event data for a defined interval that includes the traffic pattern in which the fault occurred. The goal is to show that the error slope stopped, not merely that the interface came back up.
Juniper Data Center Assurance uses DOM, BER, FEC, and CRC time-series data to identify optical-interface outliers and predict degradation. You can apply the same principle with your monitoring system: alert on counter deltas and abnormal trends, compare equivalent links, and retain the before-and-after record.
- Document the replaced component, serial or optic identifier, port, fiber route, and date.
- Save DOM readings, FEC mode, and counter snapshots before and after the repair.
- Record whether latency returned to its prior distribution and whether any queue or pause behavior changed.
- Set a follow-up review for links that remain operational but show rising corrected FEC, drifting power, or increasing bias.
Decision guide: what to do next
| If you observe | Prioritize | Avoid |
|---|---|---|
| CRC/FCS or uncorrected FEC rises on one direction | DOM comparison, compatibility check, then cable/optic/port swaps | Changing congestion policies before proving the physical path |
| PCS drops with lane asymmetry | Polarity, breakout mapping, connector path, and per-lane testing | Judging the module by a single aggregate power value |
| Queues, pause, or output drops rise with clean physical counters | Oversubscription, buffering, class-of-service, shaping, and microburst analysis | Replacing optics as the first response |
| Repeated flaps or loss-of-signal alarms | Logs on both ends, seating, optic alarms, and known-good port isolation | Clearing counters without preserving event timestamps |
| Errors persist after compatible swaps | External fiber testing and line-card, ASIC, or NIC investigation | Continuing random substitutions without a controlled record |
Common mistakes that prolong an outage
- Trusting link state: an up interface can still discard frames or add queueing delay.
- Reading lifetime totals: use timed deltas and preserve the original snapshot.
- Checking only one end: the receiving interface often contains the clearest evidence.
- Using a magic DOM threshold: trend stability, lane symmetry, alarms, and correlation with errors.
- Changing several variables: a new optic, cable, port, and FEC mode at once prevents attribution.
- Ignoring breakout details: one lane or member can fail while the aggregate interface appears mostly healthy.
- Assuming FEC hides the problem: corrected errors consume margin; uncorrected errors are already service-impacting.
- Using the wrong test instrument: select equipment for the fiber type, connector, reach, and required certification level.
When to escalate
Escalate to the cabling, hardware, or platform team when counter growth and DOM evidence remain physical after compatible component swaps, when an external test identifies loss or a break, or when errors stay tied to one port or line card. Provide the timeline, both-end counter deltas, logs, optic identifiers, DOM and lane readings, FEC configuration, swap results, and external-test report. This evidence lets the next team replace or repair the smallest proven fault domain instead of repeating the same checks.
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