25Gb links setup between two ACX routers are not coming up.  Light levels are within specification, but the interfaces remain down.  In this example, the ACX systems are an ACX7100 and an ACX7024, but the actual platform does not matter for the issue at hand.

The optic installed in both of the ACX routers is an SFP28-25G LR:

[email protected]> show chassis hardware detail no-forwarding

Hardware inventory:
Item             Version  Part number  Serial number     Description
Chassis                                ------------      JNP7024 [ACX7024]
PSM 0            REV 04   740-134837   ------------      JPSU-400W-AC-AFI
PSM 1            REV 04   740-134837   ------------      JPSU-400W-AC-AFI
Routing Engine 0 REV 09   650-136135   ------------      RE-ACX-7024
  mmcblk0 38000 MB  MMC                ------------      eMMC Disk
CB 0                      BUILTIN      BUILTIN           Control Board
FPC 0                     BUILTIN      BUILTIN           ACX7024-FPC
  PIC 0                   BUILTIN      BUILTIN           MRATE- 24xSFP28 + 4xQSFP
    Xcvr 4       0        NON-JNPR     ------------      SFP+-10G-LR
    Xcvr 23      0        NON-JNPR     ------------      SFP28-25G-BASE-LR

Each side is configured for 25g:

    et-0/0/23 {
        flexible-vlan-tagging;
        speed 25g;
        mtu 9216;
        encapsulation flexible-ethernet-services;
        ether-options {
            fec fec74;
        }
        optics-options {
            fec hgfec;
        }

Interface is down on both sides:

[email protected]> show interfaces extensive no-forwarding
...
Physical interface: et-0/0/23, Enabled, Physical link is Down
  Interface index: 1036, SNMP ifIndex: 551, Generation: 889058235077
  Link-level type: Flexible-Ethernet, MTU: 9216, LAN-PHY mode, Speed: 25Gbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Disabled, Media type: Fiber
  Device flags   : Present Running Down
  Active alarms  : LINK
  Active defects : LINK, LOCAL-FAULT
  PCS statistics                      Seconds
    Bit errors                             0
    Errored blocks                         0
  Ethernet FEC Mode  :                  FEC74

[email protected]> show interfaces extensive no-forwarding
...
Physical interface: et-0/0/12, Enabled, Physical link is Down
 Interface index: 1653, SNMP ifIndex: 529, Generation: 597000458121
 Link-level type: Flexible-Ethernet, MTU: 9216, LAN-PHY mode, Speed: 25Gbps, BPDU Error: None, Loop Detect PDU Error: None, Ethernet-Switching Error: None, MAC-REWRITE Error: None, Loopback: Disabled, Source filtering: Disabled, Flow control: Enabled, Auto-negotiation: Disabled, Media type: Fiber
 Device flags  : Present Running
 Active alarms : LINK
 Active defects : LINK, LOCAL-FAULT
 PCS statistics                     Seconds
   Bit errors                            0
   Errored blocks                        0
 Ethernet FEC Mode :                 FEC74

The recommended FEC for the 25G LR optics is FEC91:

set interfaces et-0/0/0 ether-options fec fec91

With that value set on both sides, the link came up.

2024-11-19 : Article Created

2024-11-20:  Format changes and publish