Don’t Mix Up Your FECs

Written by Craig Pasek and Pat Chou, Product Managers, Transceiver Module Group

When we introduced the 10/25G CSR (Cisco Short Reach) SFP28 transceiver (SFP-10/25G-CSR-S), we were happy to see that customers recognized its benefits right away. First, it offers longer reach than the IEEE standardized transceiver specification 25GBASE-SR. Second, it provides more options when it comes to the type of FEC (Forward Error Correction) available on its host switch, router, or server NIC (Network Interface Card). And with its higher optical performance it can still support useful reaches with weaker FEC algorithms.

One aspect of a system’s design that is often overlooked is matching the type of FEC on both ends of a link. If the host at one end supports one type, the other end had better support the same type. Otherwise, the link will not work properly. It may be nonintuitive but the choice of optic, between CSR or 25GBASE-SR, can make or break the successful operation of the link.

The FECs match

The following diagram shows a valid system configuration where the FEC type among the host settings and transceiver specifications match. In this configuration, the use of the IEEE 25GBASE-SR transceiver in the aggregation switch requires that RS-FEC is supported in the access switch and its optics.

This configuration works because FEC types is the same for all switches and transceivers.

The FECs don’t match

The following diagram shows an invalid system configuration. At each end, the FEC type available on the host is aligned with the type supported by its respective transceiver. However, the resulting FEC types at the two ends of the link do not match.

This configuration does not work because the FEC types at the two ends of the link do not match.

It should be noted that some switches only support FC-FEC (Fire Code FEC, also known as BASE-R FEC) and therefore cannot support an IEEE 25GBASE-SR transceiver because the 25GBASE-SR specification requires RS-FEC. So if the switch on the right supported only FC-FEC, then choosing a 25GBASE-SR transceiver for the NIC on the left would limit you to RS-FEC and create a mismatch.

A simple way to resolve the problem is to use CSR in the NIC instead of 25GBASE-SR. Since CSR is specified for use with either RS-FEC, FC-FEC, or even no FEC, the NIC can be set for FC-FEC. No more mismatch. See figure below for illustration.

This configuration works because the FEC types at the two ends of the link match. 10/25G CSR supports both RS-FEC and FC-FEC.

Supported FEC Types

The following table summarizes host FEC support for Cisco’s SFP-10/25G-CSR-S and Cisco’s SFP-25G-SR-S, which aligns with IEEE 25GBASE-SR.

Cisco’s SFP-10/25G-CSR-S was designed to support extended reach (300m on OM3 and 400m on OM4) applications. And since it supports FC-FEC and No-FEC (in addition to RS-FEC), it’s useful for low latency applications and can be used with early 25G switches that don’t support RS-FEC.