Port speeds and media types are evolving rapidly to address the recent need for speed because of GenAI. Configuring switch ports in Dell Enterprise SONiC is becoming an advanced undertaking. This is driven by the comprehensive new suite of hardware and software features involved.
For network admins and technicians, port configuration is not just a collection of simple settings anymore. It now requires a nuanced understanding of the interaction between protocol standards, hardware capabilities, and several interdependent software features related to them.
Below is an in-depth technical introduction to the configuration space, software features and CLI examples, for the most relevant port features: speed, forward error correction (FEC), media port auto-configuration, auto-negotiation, link training, and the differences arising from copper vs. fiber implementations.
- Port Speed Configuration
Port speed directly determines the data rate supported by the interface. Dell PowerSwitch platforms with Enterprise SONiC support a wide matrix of speeds per platform — from as low as 10M or 100M to 1G, 10G, 25G, 40G, 50G, 100G, 200G, 400G and even 800G. The speed setting is tightly coupled to the installed transceiver (or direct-attach cable), platform ASIC, and in breakout scenarios, logical port mapping.
CLI Example
To set a fixed speed for a port in the Management Framework CLI:
Sonic (config-if-Ethernet68)# speed 10000
sonic(config-if-Ethernet68)# no shutdown
Speed Auto negotiations using Speed Auto
An Ethernet interface autonegotiates speed with a connected device so that both transmit data at the highest set of common capabilities. Both sides of a link must have autonegotiation enabled or disabled for the link to come up. When enabled, autonegotiation automatically configures interface speed over twisted-pair and copper links. Autonegotiation is enabled by default only on Copper RJ45 ports.
Note: Not to be confused with Autoneg, which is available on management interfaces (Out of band management) as explained in the coming section below.
CLI Example
Enable auto-negotiation on a port:
sonic(config-if-Ethernet1/1)# (no) speed auto
- Autoneg
An Ethernet interface (mostly on out of band management interface) autonegotiates speed, duplex setting, …etc. with a connected device so that both transmit data at the highest set of common capabilities. Both sides of a link must have autonegotiation enabled or disabled for the link to come up. When enabled, autonegotiation automatically configures interface speed over twisted-pair and copper links (out of band management).
CLI Example
Enable autoneg on an out of band management interface
sonic(config-if-Management0)# (no) autoneg on
Note: Not to be confused with Speed Autonegotiation described in a previous section.
- Forward Error Correction (FEC)
FEC enhances link reliability, especially over high speed 25G/100G NRZ and over 200G/400G/800G PAM4 links subject to error rates due to signal integrity issues. Dell Enterprise SONiC supports manual configuration of FEC modes such as RS (Reed-Solomon), FC (FireCode), or disabling FEC entirely, based on media and platform capabilities.
CLI Example
To set the FEC mode:
sonic(config-if-Ethernet1/1) FEC RS
Forward error correction auto mode
FEC-auto (Forward Error Correction auto mode) in Dell SONiC allows the switch to identify the locally connected transceiver and determining its preferred FEC setting by automatically evaluating its hardware capabilities and its media type.
When FEC-auto is enabled, the switch queries the plugged in transceiver. Then the switch can identify which FEC algorithms (such as BASE-FEC/Clause 74 or RS-FEC/Clause 91) are supported and optimal for its interface and data rate. CAUTION: it is possible link may not come up due to mismatched FEC setting between the two link partners.
If Auto-negotiation is turned on (speed auto and not autoneg as per the above sections), this detected preference is advertised during link initialization. Then auto-negotiation sets appropriate capability bits in accordance with Ethernet standards. All that occurs without requiring manual intervention. As a result, the local transceiver transparently communicates its most suitable FEC setting to the link partner, maximizing link performance and reliability under the current deployment conditions, while reducing the risk of misconfiguration or suboptimal FEC selection that could arise from manual settings.
CLI Example
To set the FEC auto mode:
sonic(config-if-Ethernet1/1) FEC-Auto
Media/Platform Notes
- Some optical transceivers such as the QSFP28 100G BIDI optic necessitate FEC OFF for the link to come up. If FEC is on, the link stays down. Below is a list for the optical transceivers, which necessitate FEC OFF as detailed in the lab testing results table below.
- 100G-SR1.2,
- 100G-BIDI,
- 100G-ER4-LITE (keep FEC ON only if Engineered SMF link distant is between 30km to 40km, otherwise keep FEC OFF for SMF link distant up to 30km)
- 100G-FR1,
- 100G-LR1
- QSFP56-DD or 4x100G breakouts may require FEC=RS, particularly with ACC/AOC or DAC cables on S5448F – ON—failure to configure accordingly causes link establishment failures.
Note: All the transceivers identified in red colors necessitate avoiding FEC auto.
- Link Training
Link training is critical for high-speed direct-attach copper (DAC). It involves dynamic adjustment of transmitter/receiver parameters (preemphasis, equalization, etc.) to optimize signal integrity. In Dell SONiC, link training can be controlled independently from auto-negotiation as standalone link training.
Stand-Alone Link Training
If auto-negotiation is not needed, standalone link training may be enabled for signal optimization (e.g., between fixed-speed endpoints). However, when auto-negotiation is on, link training is performed as a step in the process. CAUTION: standalone link-training should be disable on media that is not a passive DAC cable.
CLI Example
To enable standalone link-training:
sonic(config-if-Ethernet1/1) (no) standalone-link-training
- Media-Based Port Autoconfiguration
SONiC’s media-based autoconfig mechanism automatically detects installed transceivers and adjusts port parameters such as power, supported speeds, and sometimes default FEC settings. However, some critical aspects — including FEC and link training, must often be manually aligned to the deployed cable/optic and the platform’s operational recommendations.
- Detailed Copper vs. Fiber Optic Port Complexities
- RJ45 Base-T (1G/10G) ports require auto-negotiation for link establishment. Manual speed/duplex settings are not recommended; links often remain down without negotiation.
Example: “speed auto” is critical for ports on Z9100-ON, S5232F-ON, etc. Even multigig copper SFP modules (e.g., 10GBASE-T) may have platform-specific negotiation or power constraints.
Note: On certain platforms (N/E3248 series), auto-negotiation is not supported at all in SFP ports with copper SFPs — use explicit speed configuration or consult compatibility matrices for device support.
- Optics transceiver, active copper cable (ACC), or active electrical cable (AEC) do not support auto-negotiation and/or standalone link-training at all — configuration is manual for speed and FEC. For instance, QSFP28 100G BIDI links demand FEC is disabled on both ends for the link to come up.
- Conclusion
Advanced SONiC port configuration intertwines detailed knowledge of platform hardware, Dell qualification guidelines and careful CLI operation. Misconfiguration of key features (speed, FEC, negotiation, link training, power) can lead to links remaining down or running suboptimally, especially when deploying a mix of copper and fiber or using breakout modes. Reference to per-platform matrices and thorough diagnostic use of show commands is essential to robust port deployment in production. Dell’s enterprise enhancements, including Management Framework and detailed compatibility matrices, provide essential tools for the technical practitioner to deploy at scale and maintain high reliability.
