Overview

When deploying high-speed 100G Ethernet links using QSFP28 optics, encountering a Forward Error Correction (FEC) mismatch is a common yet critical issue that can severely impact throughput and link stability. This FAQ is tailored for B2B network architects and telecom engineers who need immediate, authoritative answers on diagnosing and resolving FEC mismatches, alongside broader pre-sales and post-sales technical queries for high-performance routing platforms. Below, we address the root causes of FEC errors, CLI verification steps, and broader integration topics from transceiver compatibility to redundant power failover.

Frequently Asked Questions

Q1: Why does my router report a FEC mismatch on a 100G QSFP28 optical link?

The FEC mismatch error is definitively caused by an incompatibility between the FEC modes configured on the router's port and the FEC capabilities (or lack thereof) of the connected transceiver or line card. FEC, such as Reed-Solomon RS(528,514) or Base-R, is essential for maintaining a low Bit Error Rate (BER) over 100G links. When the router expects a specific FEC type (e.g., 'cl91' or 'rs-fec') and the optic is set to a different standard or is disabled, the link will not achieve a healthy state. This is commonly seen when mixing third-party optics or connecting to legacy switches that do not support the same FEC standard as defined by IEEE 802.3bj for 100GBASE-SR4 or 100GBASE-LR4.

Q2: How do I configure FEC settings via CLI to resolve a mismatch on a 100G interface?

To resolve the mismatch, you must first verify the current FEC status using the CLI command 'show interfaces ethernet capabilities' to see supported FEC modes. Then, administratively shut down the interface, change the FEC mode to match the optic standard using 'fec {rs-fec | base-r | none}' under the interface configuration, and re-enable the port. For example, if the optic is a standard 100GBASE-LR4, you would typically set 'fec rs-fec'. After applying, verify with 'show interfaces ethernet transceiver details' to confirm the FEC negotiation success and monitor the corrected BER counters to ensure stability.

Q3: What is the maximum switching throughput and backplane capacity of a typical 100G-capable core router?

The maximum switching capacity for a high-end modular core router supporting 100G QSFP28 ports is typically between 12.8 Tbps and 25.6 Tbps, with a packet forwarding performance of up to 5.6 billion packets per second (Bpps) for 64-byte frames. This line-rate performance is driven by advanced ASIC architectures that process data at wire speed across all slots, leveraging a high-speed backplane operating at 1.6 Tbps per slot to ensure that all QSFP28 ports can simultaneously handle full-duplex 100G traffic without contention.

Q4: Are third-party 100G QSFP28 transceivers compatible, and can they cause FEC errors?

Yes, third-party 100G QSFP28 transceivers are often physically compatible, but they are a frequent source of FEC mismatches and link errors. While many modern routers support third-party optics via a 'transceiver permit' command, the EEPROM programming on some generic modules may default to non-standard FEC configurations or fail to advertise their capabilities correctly to the router's software. To ensure compatibility, either procure optics with 'generic' firmware that matches your router's FEC expectations or use officially validated modules from your hardware vendor's compatibility matrix to ensure both RS-FEC and Base-R modes are supported.

Q5: What is the primary performance difference between QSFP28 ports and previous generation QSFP+ ports?

The primary performance difference lies in the data rate and the mandatory use of FEC. While QSFP+ supports up to 40G (4x10G) without requiring FEC for typical link lengths, the QSFP28 supports 100G (typically 4x25G or 2x50G) and relies heavily on RS-FEC to maintain signal integrity at these higher speeds. Additionally, QSFP28 ports often support a wider range of breakout cables (e.g., 100G to 4x25G SFP28) and are designed with stricter thermal and power budgets (typically under 3.5W) compared to QSFP+ modules.

Q6: How can I verify if a FEC issue is causing latency or packet drops on my 100G link?

You can verify the impact of FEC errors by checking the interface statistics for 'uncorrected FEC errors' and 'corrected FEC error counters' using the CLI. A high rate of corrected FEC errors indicates marginal link quality that can introduce slight latency due to the error correction processing, while uncorrected errors will result in cyclic redundancy check (CRC) drops and retransmissions. Use the command 'show interfaces ethernet fec' to view corrected and uncorrected FEC error counts; if the corrected error rate exceeds 1e-6, you should re-evaluate the physical layer (cables, connectors, or optic power).

Q7: What are the CLI commands to enable stacking interfaces for managing multiple high-speed chassis?

To enable stacking on supported modular routers with 100G uplinks, you must first designate dedicated physical QSFP28 ports as stacking ports using the global command 'stack stack-id member priority '. Then, under the interface configuration, apply 'stack-port enable' and 'stack-port group '. For virtual chassis, you must also set the unit ID and re-number members using 'switch renumber' and 'stack-unit stack-group ' followed by a system 'copy running-config startup-config' and a reload to establish the stacking topology.

Q8: How does the router handle redundant power supply failover, and how can I troubleshoot a failover event?

The router handles redundant power supply failover via an active-standby or load-sharing architecture, utilizing a dual AC/DC input system with a hot-swappable design. Upon failure of the primary PSU, the secondary unit takes over instantly without rebooting the system, supported by the backplane's power distribution. To troubleshoot a failover event, review the system logs using 'show logging' for power-related alerts (e.g., 'PSU-2: Input lost'), check the physical status with 'show environment power', and ensure that the failed unit's circuit breaker and input voltage are functioning. If a failover occurs repeatedly, it may indicate an overcurrent condition or a faulty PSU that requires immediate replacement.