Troubleshoot Interface CRC Errors on IOS XR Routers

Introduction

This document describes how to troubleshoot Cyclic Redundancy Check (CRC) errors on interfaces within Cisco IOS XR routers. Cisco recommends knowledge of the Cisco IOS XR platform and admin CLI access. Applicable platforms include ASR 9000 Series Routers (ASR 9006, ASR 9010), NCS 540/560/5500/5700 Series Routers.

CRC errors are one of the most common ticket types in any service provider or large enterprise network, and they are also one of the most frequently misdiagnosed. The counter is trivial to read and almost impossible to interpret in isolation. This expanded guide keeps the Cisco procedure intact, then adds the physics behind the counter, the commands that produce usable evidence, a worked case study, and an operational model for catching errors before they become customer-visible.

Problem

Cisco IOS XR platforms leverage CRC checks on physical interfaces (Ethernet, optical, and so on) to maintain link reliability, and provide interface statistics that include CRC error counters. High CRC error counts typically indicate physical layer issues such as faulty cables, connectors, or transceivers. On platforms like the NCS 5500/5700 Series and ASR 9000 Series, CRC error trends can trigger alarms or automated workflows to minimize downtime.

Common Causes of Interface CRC Errors

• Configuration mismatches: such as Maximum Transmission Unit (MTU) size mismatches between devices, which can cause large packets to be truncated incorrectly, leading to CRC errors.
• Physical media faults: damaged fiber, copper or DAC cables, dirty connectors, or failing transceivers.
• Dirty or mated-once optical connectors: a single fingerprint on a duplex LC ferrule can scatter enough light to push the receiver below its sensitivity threshold.
• Marginal optical power: a link running with almost no receive margin will pass clean traffic for weeks, then start corrupting frames as ambient temperature drifts.
• Speed, duplex or FEC negotiation mismatches: auto-negotiation falling back to a different duplex or an RS-FEC mismatch on 25G/100G ports breaks frame integrity end to end.
• Bad patch panel or cross-connect: a bent patch cord, a crushed jumper or a damaged MPO trunk inside a cable tray.
• Hardware defects: a failing optics cage, a marginal PHY on a line card, or backplane signalling problems between the line card and the fabric.
• Electromagnetic and grounding issues: mostly on copper (1000BASE-T, 10GBASE-T and DAC) where shield currents and poor bonding inject noise.

What a CRC Error Actually Means on the Wire

An Ethernet frame ends with a 32-bit Frame Check Sequence (FCS). The sender computes that checksum over the destination MAC, source MAC, length/type, payload and padding using the CRC-32 polynomial (0x04C11DB7, transmitted bit-reversed). The receiver recomputes the same polynomial as the bits arrive and compares the result. If the two disagree, the frame is discarded at the MAC layer and the interface increments its CRC / input-error counter.

Three properties of that mechanism drive the entire troubleshooting method:

1. The frame is dropped in hardware. The forwarding engine never sees it. A CRC error therefore cannot be caused by routing, ACLs, QoS policies, MPLS labels or a control-plane problem, no matter how tempting it is to look at the config first.
2. The counter is blind to the source. It records that bits changed in flight, not which component changed them. Every diagnosis is therefore an exercise in bisection: narrow the suspect list by substitution until the counter stops increasing.
3. Corruption is often probabilistic. A marginal optical link may corrupt one frame in ten million. That is invisible in a one-second sample but obvious over a fifteen-minute interval, which is why rate matters more than absolute count.

CRC vs Other Input Errors

A useful rule of thumb: if CRC rises together with symbol errors and FEC counters, the problem is in the optical or electrical signalling path. If CRC rises alone while FEC stays clean, suspect framing — duplication inside a Layer 2 loop, a mirrored/miscabled link, or a software defect on the forwarding path. That single split saves hours.

Reading IOS XR Counters Correctly

Start with the interface itself. On IOS XR the familiar Cisco IOS counters are replaced by a different set of show commands, and reading only one of them is a common mistake.

show interfaces TenGigE0/0/0/1
show interfaces TenGigE0/0/0/1 accounting
show interfaces TenGigE0/0/0/1 rates
show controllers TenGigE0/0/0/1
show controllers TenGigE0/0/0/1 internal
show controllers TenGigE0/0/0/1 phy
show interfaces TenGigE0/0/0/1 detail

In show interfaces the relevant block looks like this:

Five minute input rate 0 bits/sec, 0 packets/sec
Five minute output rate 0 bits/sec, 0 packets/sec
    0 packets input, 0 bytes, 0 total input drops
    0 drops for unrecognized upper-level protocol
    Received 0 broadcast packets, 0 multicast packets
            0 runts, 0 giants, 0 throttles, 0 parity
    0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort

For clearing a counter you must use the correct syntax or nothing happens; on IOS XR it is:

clear counters TenGigE0/0/0/1

Clearing resets the hardware and software counters, so always record the timestamp and the value you cleared. A counter that has grown for 400 days tells you nothing actionable; a counter that gains 12 000 CRC errors in five minutes tells you the link is actively broken.

Turning Counts into a Rate

The single most useful diagnostic is a two-sample rate measurement. Take a baseline, wait five to fifteen minutes under production load, then take a second sample; an idle link will under-report, because marginal optics corrupt only when the PHY is exercised.

show interfaces TenGigE0/0/0/1 | include "CRC"
! wait 15 minutes
show interfaces TenGigE0/0/0/1 | include "CRC"

Thresholds worth automating: a hard alarm at any non-zero growth in a five-minute window on a customer-facing interface, and a warning at any non-zero growth on 400G interfaces, where FEC is expected to absorb all errors before they reach the CRC counter.

Procedure to Resolve Interface CRC Errors

1. Verify MTU Settings

Check the MTU configured on the interface of the local router and the connected peer device. While less common for CRC errors than physical issues, an MTU mismatch can sometimes lead to frame truncation and subsequent CRC errors.

show interfaces TenGigE0/0/0/1 | include MTU
show interfaces TenGigE0/0/0/1 mtu

Remember that IOS XR reports MTU excluding the Layer 2 header on most interfaces, while many peers report payload MTU, so a 9114 on one side and 1500 on the other is not automatically a mismatch — verify with an end-to-end ping carrying a known payload size rather than comparing numbers across vendors.

2. Inspect and Replace Physical Media

• Visually inspect the cable (fiber, copper, DAC) for any damage, kinks, or sharp bends.
• Ensure the cable is securely seated in both the router interface and the connected device.
• Action: replace the cable with a known good one.
• Clean both ends with a one-click or cassette cleaner; never reuse a dirty end or touch a ferrule with a bare finger.
• Check the patch panel and any cross-connect in the path — the failure is frequently in a jumper you did not install and cannot see.
• Confirm the transceiver part number and reach match the plant: a 10 km optic in a 40 km span will run with negative margin from day one.

3. Use External Loopback

Use external loopback sensibly to identify the hardware that is causing CRC. If CRC errors stop, the issue is likely further upstream (for example, remote device, cable). If they continue, the transceiver or the port hardware is suspect.

controller Optics0/0/0/1
 loopback internal
 commit

Loop the port with a known-good fibre jumper or a QSA/loopback plug, clear the counters, send a defined amount of test traffic (a repeated ping of 1400 bytes at line rate is enough on most platforms), and read the counters again. Because the loopback removes the cable plant from the equation, this test cleanly separates "inside the chassis" from "outside the chassis".

4. Move Interface to a Different Port/Line Card

• If possible, move the cable and transceiver to a different port on the same line card. If errors persist, the line card itself can be faulty.
• If errors stop, the original port was likely defective.
• If the same transceiver follows the errors to a new port, the transceiver is the culprit; if the errors stay with the original port regardless of transceiver, the port is the culprit.
• Move a spare transceiver into the original port as the control experiment — a single swap proves nothing on its own.

5. Check Known Software Issues

• Search the Cisco Bug Search Tool (BST) for your platform, interface type, and software version.
• Review Cisco Field Notices, Release Notes, and known Caveats.
• Pay particular attention to defects in the optics driver or the fabric code path, which can report CRC when the real fault is in the ASIC or in FEC handling.
• Record the exact XR release (including SMU level) in the ticket; CRC defects are frequently fixed by an SMU rather than a full upgrade.

Case Study: 100GE Uplink Corrupting One Frame in Ten Thousand

A production 100GE uplink between an NCS 5500 and a peer router showed no alarms but a latency-sensitive application reported retransmits. Investigation proceeded as follows.

1. Counters. show interfaces HundredGigE0/0/0/2 showed 41 288 input errors with 41 190 CRC over 38 days, and show controllers ... phy showed pre-FEC BER at 3.1e-6 with uncorrectable FEC blocks beginning to appear. Rx power was 1.9 dB above the specified sensitivity, which looked acceptable at first glance.
2. Rate measurement. A fifteen-minute sample showed 1 842 CRC, roughly one corrupted frame per 12 000 — consistent with the application's retransmit alerts and invisible in any one-second counter sample.
3. Bisection. An internal loopback on the port produced zero CRC, which excluded the line card and the transceiver's transmit path. Moving the far-end jumper to a spare fibre strand dropped CRC to zero within ten minutes.
4. Root cause. A dirty duplex LC connector in an intermediate patch panel; the contaminated ferrule cost roughly 2 dB of receive margin, which was invisible on a cold morning and fatal once the optics warmed up in the afternoon.
5. Fix. Cleaned and re-terminated the jumper, added a receive-power alarm at 3 dB above sensitivity rather than 1 dB, and enabled periodic pre-FEC BER trending.

The lesson generalises: pre-FEC BER and FEC counters are the early-warning system, and CRC is what you see after the warning was ignored.

Optical-Specific Checks: dBm, FEC and Coherent Alarm Data

On optical and coherent interfaces, the controller data matters more than the interface counters. Gather it in one pass before touching anything, so you have a baseline to compare after each change.

show controllers Optics0/0/0/1
show controllers Optics0/0/0/1 phy
show controllers Optics0/0/0/1 internal
show interfaces HundredGigE0/0/0/1 fec statistics
show alarm optics

Read the transmit and receive power against the datasheet numbers for that specific optic, not against a remembered rule of thumb, and read OSNR and pre-FEC BER alongside them. A link that is 10 dB inside its power budget but with a pre-FEC BER of 1e-5 has a signalling or reach problem, not a power problem, and swapping optics will not fix it. The same counters on a router with a broken fan tray, incidentally, will show a slow power drift as the optics heat — check the environmental alarms before declaring the transceiver faulty.

Automating CRC Detection Before Customers Notice

Polling free counters on a schedule and alarming on the delta catches degraded links days before they fail. The pattern below uses IOS XR model-driven telemetry over gRPC so the router pushes counter changes rather than waiting to be polled.

telemetry model-driven
 destination-group DG-1
  address family ipv4 10.10.10.50 port 57000
   encoding self-describing-gpb
   protocol grpc no-tls
  !
 !
 sensor-group SG-INTF
  sensor-path Cisco-IOS-XR-infra-statsd-oper:infra-statistics/interfaces/interface/latest/generic-counters
 !
 subscription SUB-INTF
  sensor-group-id SG-INTF sample-interval 30000
  destination-id DG-1
 !
!

Pair that with a simple script that keeps a per-interface baseline and raises an alert whenever the CRC, symbol-error or FEC-uncorrectable fields increase inside a window. Store the baseline on disk so the check survives a restart of the collector.

import time, json, pathlib

BASE = pathlib.Path("crc_baseline.json")
baseline = json.loads(BASE.read_text()) if BASE.exists() else {}

while True:
    for name, counters in poll_interfaces():
        prev = baseline.get(name)
        if prev and counters["crc"] > prev["crc"]:
            alert(f"{name}: +{counters['crc'] - prev['crc']} CRC in window")
        baseline[name] = counters
    BASE.write_text(json.dumps(baseline))
    time.sleep(300)

Prevention: Design and Operational Practices

• Use LACP bundles, not single links. A member with rising CRC can be administratively shut while the bundle keeps forwarding; a single link can only fail.
• Budget receive margin deliberately. Aim for at least 3 dB above the optic's specified sensitivity at end of life, and alarm on the trend rather than only on the absolute value.
• Trend, do not threshold. Pre-FEC BER, symbol errors and CRC deltas are leading indicators; a static "CRC > 100 = critical" threshold is either too late or permanently noisy.
• Keep a spare of every optic SKU in the rack, with the correct reach and connector type, plus a cleaning kit at each end of the link.
• Document the fibre path. When a fault appears, knowing that there are three patch panels and a cross-connect between two routers converts a two-day hunt into a five-minute jumper swap.
• Clean before you blame. Contamination is the single most common root cause of marginal CRC on a link that was working yesterday.

Quick Checklist

1. Capture a rate sample, not a raw count, and record the interval.
2. Break input errors down into CRC, runts, giants and overruns — they mean different things.
3. Compare local and remote counters; the side that increments owns the problem, in the receive direction.
4. Read pre-FEC BER and FEC counters before touching hardware.
5. Loop the port internally to test the chassis.
6. Substitute transceiver, then cable, then port, one variable at a time.
7. Clean and reseat both ends of the optical path.
8. Check BST, field notices and SMU history for the exact platform and release.
9. After the fix, clear counters and re-measure over a window that includes peak load.
10. Add the interface to delta-based monitoring so the next degradation is caught early.

For complementary material on the same layer, see our IOS XR CRC troubleshooting walkthrough, the Junos error field reference, and the SFP transceiver troubleshooting checklist.

Products Covered

• ASR 9000 Series Aggregation Services Routers
• Network Convergence System 500/540/560/5000 Series Routers
• NCS 5500/5700 Series Routers

原文链接：https://www.cisco.com/c/en/us/support/docs/routers/asr-9000-series-aggregation-services-routers/225385-troubleshoot-interface-crc-errors-on.html