Forward Error Correction

Table 1. Forward Error Correction product support
Feature	Product	Release introduced
Forward Error Correction (FEC) (configurable)	5320 Series	Not applicable
	5420 Series	VOSS 8.4 5420M-24T-4YE, 5420M-24W-4YE, 5420M-16MW-32P-4YE, 5420M-48T-4YE, and 5420M-48W-4YE
	5520 Series	VOSS 8.2.5 5520-VIM-4YE only

Forward Error Correction (FEC) is a method of obtaining error control in data transmission over an unreliable or noisy channel in which the source (transmitter) encodes the data in a redundant way by using an error correcting code (ECC). This redundancy enables a destination (receiver) to detect a limited number of errors and correct them without requiring a re-transmission.

FEC is useful where re-transmitting data is either expensive or impossible, for example, when transmitting to multiple receivers in multicast. However, although FEC provides more error control, it introduces a latency in data transmission.

FEC Configuration Options

You typically configure FEC on a port. The supported options are:

cl91 (Clause 91 RS-FEC)
cl108 (Clause 108 RS-FEC):
cl74 (Clause 74 Firecode R-FEC):
auto:

FEC is not supported on:

Out-of-band (OOB) management ports.
100 GbE ports that are changed to 40 GbE ports by dynamically swapping 100 Gb modules with 40 Gb modules. FEC does not support the 40 Gbps speed.

Important

On ports that support FEC configuration, ensure that you configure the same option at both end-points. Otherwise, the link does not come up.
You must enable FEC to achieve proper functionality when using interconnects such as the 25Gb SR, 25 Gb SR-lite, 25 Gb ESR optics or the 25 Gb AOC and 25 Gb DAC.
FEC is not required on 100 Gb or 25 Gb long-range optics because these optics do error checking internally.

Clause 91 RS-FEC

This option supports both the 25 Gbps and 100 Gbps speeds. You can configure this option on ports with either the 100GbSR4 or 100GbCR4 modules plugged in, or on 100 GbE channelized ports operating at 25Gbps speed.

Note

Ensure that you enable Auto-Negotiation for ports with the 100GbCR4 modules plugged in; it is mandatory.

Clause 108 RS-FEC

This option also supports both the 25 Gbps and 100 Gbps speeds. It is similar to Clause 91 but provides extra latency.

Clause 74 Firecode R-FEC

This option supports only the 25 Gbps speed and is used in applications that require reduced latency.

Auto

This option automatically configures FEC based on port speed and pluggable module type.

For 25 Gbps speeds, FEC CL108 is enabled for all transceiver types.
For 100Gbps speeds:
- FEC is disabled for 100GbE LR4 and ER4 transceivers.
- FEC CL91 is enabled for all other transceiver types (for example, 100GbE SR4, CR4, AOC, CWDM4, SWDM4).

FEC and Auto-Negotiation

FEC is a negotiated port attribute for 25 Gb and 100 Gb connections that support Auto-Negotiation. If you enable Auto-Negotiation on a port for a supported transceiver type, the switch uses the configured FEC value in the negotiation advertisement. Peers can advertise different values, which means the resulting FEC operational state can be different than the one advertised.

The following table lists the 25 Gb end-point advertisements and the resulting FEC operational state:

Table 2. 25 Gb end-point advertisements
Peer A	Peer B	Result
CL108	CL108	CL108
CL74	CL74	CL74
No FEC	No FEC	No FEC
No FEC	CL108	CL108
No FEC	CL74	CL74
CL74	CL108	CL108

The following table lists the 100 Gb end-point advertisements and the resulting FEC operational state:

Table 3. 100 Gb end-point advertisements
Peer A	Peer B	Result
CL91	CL91	CL91
No FEC	No FEC	CL91 Note: Even when both peers advertise no FEC, negotiation results in clause 91 FEC per IEEE standard mandatory setting.
No FEC	CL91	CL91

You can use the show interfaces gigabitEthernet config command to see the FEC operational state for a port.

For additional details about support, see Default Auto-Negotiation Behavior.