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fec (ether)


Hierarchy Level


(PTX10001-36MR, PTX10003-8T, PTX10003-16T, PTX10004, PTX10008, QFX5220-32CD, and QFX5220-128C) Enable or disable RS-FEC (Reed-Solomon Forward Error Correction) for 25, 50, 100, 400-Gigabit Ethernet interface. By default, the Junos OS software enables or disables forward error correction based on the plugged-in optics. For instance, Junos OS software enables RS-FEC for 25G or 50G SR4 optics and disables RS-FEC for 25G or 50G LR4 optics.

If you enable or disable RS-FEC using this statement, this behavior applies to any 25-Gigabit Ethernet or 50-Gigabit Ethernet optical transceiver installed in the port associated with the interface.

You can configure forward error correction (FEC) clauses CL74 on 25-Gigabit and 50-Gigabit interfaces, and CL91 on 25-Gigabit or 50-Gigabit interfaces. Since the FEC clauses are applied by default on these interfaces, you must disable the FEC clauses if you do not want to apply them.

FEC mode is assigned by default. If there is a FEC mismatch, the link between nodes can go down. You must disable FEC mode if you do not want it assigned by default.

To view the FEC mode on an interface, use the show interfaces interface-name command.

For instance, consider two peer nodes, Node1 and Node2. Node1 is currently running Junos OS Evolved release 21.1R1 and the peer node (Node2) is currently running any Junos OS Evolved release prior to 21.1R1. The link between the two nodes will go down because of FEC mismatch. To enable the link to come back up, you must manually change the FEC on any one node by using the fec option at the [edit interfaces <interface-name> ether-options] hierarchy level.

Default FEC for prior to 21.1 Junos OS Evolved 21.1 is FEC74 and from 21.1 Junos OS Evolved is FEC91.

When FEC mode is FEC74 or FEC91 or NONE for all channels, FEC gets locked and interface is UP. For example, when FEC91 is configured together with FEC74 or NONE, on channel-0 and 1 or channel-2 and 3, link goes down for both channels.

There is no dependency between Channel 0(1) and channel 2(3). FEC74 can be configured with NONE to any interface et-x/y/x:(0/1/2/3) and link comes UP leads to the following:

  • When you configure FEC91 on interface-x/y/z:0 then you must configure FEC91 on interface- x/y/z:1.
  • When you configure FEC91 on interface-x/y/z:2 then you must configure FEC91 on interface- x/y/z:3.
  • When you configure FEC74 or NONE on interface-x/y/z:0/1 then you must configure FEC74 or NONE on interface- x/y/z:2/3.



IEEE 802.3by Clause 108, Reed-Solomon FEC (RS-FEC) on 25g links


FEC74 enabled


IEEE 802.3bj Clause 91, Reed-Solomon FEC (RS-FEC)


FEC disabled


Junos OS software automatically enables or disables 25-Gigabit Ethernet and 50-Gigabit Ethernet for FEC91 based on the type of pluggable optics used.

Required Privilege Level

interface—To view this statement in the configuration.

interface-control—To add this statement to the configuration.

Release Information

Statement introduced in Junos OS Release 16.1R1