How does Link Fault Pass-Through work?

Link Fault Pass-Through (LFP), also referred to as LFPT, is a media converter feature that actively propagates a link failure from one network interface to the other — ensuring that upstream switches and devices receive a link-down signal and can trigger redundancy switchover, rather than silently discarding traffic on a failed path. Without LFP, a media converter masks link failures on one side, causing connected devices on the other side to continue transmitting frames that are never delivered — a condition that can result in minutes or hours of silent data loss before the fault is manually detected. ORing industrial media converters ([MC_PRODUCT_MODELS]) support LFP with a reaction time of [REACTION_TIME], enabling integration with STP, RSTP, O-Ring, O-Chain, and other link-failure-triggered redundancy protocols.

The problem: media converters that hide link failures

A media converter connects two different physical media — typically Ethernet copper (RJ-45) and optical fibre (LC or SC) — converting electrical signals to optical and vice versa. In normal operation, frames flow transparently in both directions. The problem arises when one side of the media-converted link fails.

Consider a typical deployment: a managed switch connects to a media converter via RJ-45, and the media converter connects to a remote fibre link. If the fibre cable is cut or the remote transceiver fails:

The media converter detects Loss of Signal (LOS) on its optical port
On the Ethernet side, however, the media converter keeps the RJ-45 port in link-up state
The managed switch's port status shows Connected — it has no indication of the fibre failure
The switch continues transmitting frames toward the failed fibre path
All transmitted frames are silently discarded at the media converter — they cannot be delivered
No link-down event occurs, so STP/RSTP, O-Ring, and other redundancy protocols are never triggered

The result is a network outage that appears — from the switch's perspective — as a healthy link. Traffic is lost with no automatic recovery until an operator physically identifies and resolves the fault.

How Link Fault Pass-Through works

Link Fault Pass-Through solves this problem by actively monitoring both network interfaces and immediately forcing a corresponding link-down state on the opposite side when a failure is detected on either side.

Fibre failure → Ethernet side propagation

When the optical port detects Loss of Signal (LOS) — caused by a fibre cut, dirty connector, transceiver failure, or remote device power-off:

The media converter detects LOS on the optical port
Within [REACTION_TIME], the media converter forces the Ethernet (RJ-45) port into a link-down state
The connected switch detects the link-down event on its port
The switch triggers its configured redundancy protocol (STP, RSTP, O-Ring, O-Chain) to reroute traffic via an alternative path
When the fibre link is restored, the media converter re-enables the Ethernet port and the switch re-establishes the link via autonegotiation

Ethernet failure → fibre side propagation

When the Ethernet port detects a link-down event — caused by the connected switch port going down, the RJ-45 cable being unplugged, or the switch itself powering off:

The media converter detects link-down on the Ethernet port
The media converter immediately stops transmitting on the optical fibre side
The remote end detects Loss of Signal on its optical receiver
The remote device (switch or media converter) triggers its redundancy protocol

This bidirectional propagation ensures that any failure — at either end of the media-converted link or in the fibre path itself — is visible to all connected devices within [REACTION_TIME].

Link Fault Pass-Through diagram showing a media converter propagating a fibre-side link failure to the Ethernet side (forced link-down) and an Ethernet-side failure to the fibre side (stop transmitting), enabling upstream switches to trigger redundancy switchover — LFP bidirectional failure propagation: a fibre failure forces Ethernet link-down (left to right); an Ethernet failure stops fibre transmission (right to left). Both directions trigger upstream redundancy protocols.

With LFP vs without LFP — comparison

Scenario	Without LFP	With LFP
Fibre link fails	Ethernet port stays up — switch sees no failure	Ethernet port forced down — switch detects failure within [REACTION_TIME]
Ethernet port fails	Fibre side keeps transmitting — remote end sees no failure	Fibre transmission stopped — remote end detects LOS within [REACTION_TIME]
Redundancy protocol (STP/O-Ring) response	Not triggered — no link-down event received	Triggered immediately — link-down event received from media converter
Data loss during failure	Continuous — frames discarded silently until manual intervention	Minimised — traffic reroutes via redundancy path within protocol convergence time
Failure detection method	Manual — operator notices outage and investigates	Automatic — link-down event detected by switch within [REACTION_TIME]
MTTR (Mean Time to Restore)	Minutes to hours — depends on operator response	Milliseconds to seconds — depends on redundancy protocol convergence

LFP and network redundancy protocols — how they work together

LFP is a Layer 1 (physical layer) mechanism — it passes electrical and optical link state signals through the media converter. Network redundancy protocols such as STP, RSTP, O-Ring, and O-Chain operate at Layer 2 (data link layer) and require a Layer 1 link-down event to trigger their convergence algorithms.

LFP and redundancy protocols are complementary, not alternatives:

Without LFP: redundancy protocols cannot detect failures masked by media converters — they never receive the link-down trigger they need
With LFP: redundancy protocols function exactly as designed — the link-down signal passes through the media converter and reaches the switch, triggering convergence

In any network topology that uses media converters in combination with STP, RSTP, MSTP, O-Chain, or other link-failure-triggered protocols, LFP is a required feature — not an optional enhancement.

Industrial use cases for Link Fault Pass-Through

Fibre ring topologies in power substations

Power substation automation networks (IEC 61850) frequently use fibre ring topologies with media converters at each IED or bay controller. If a fibre segment breaks and LFP is not enabled, the ring's redundancy protocol will not detect the break and will not reroute traffic — potentially causing protection coordination failures. With LFP enabled, the break is immediately visible to all switches in the ring, and the redundancy protocol restores connectivity in milliseconds.

Long-distance fibre links in CCTV and surveillance networks

Surveillance networks in tunnels, highways, and industrial facilities often use single-mode fibre for long-distance segments with media converters at each camera cluster. A fibre cut between two media converters would — without LFP — leave the VMS (Video Management Server) believing the cameras are reachable and waiting indefinitely for frames that will never arrive. With LFP, the VMS switch immediately receives a link-down event and can alert operators to the specific segment that failed.

Factory automation with copper-to-fibre conversion

Factory automation networks connecting PLCs, HMIs, and SCADA servers may use media converters to extend beyond the 100-metre limit of Cat5e/Cat6 copper cabling. In ring or star topologies with redundancy, LFP ensures that any fibre failure or media converter power loss is immediately propagated to the managed switches, activating the configured redundancy path and maintaining production continuity.

Frequently asked questions

What is Link Fault Pass-Through (LFP)?

LFP (Link Fault Pass-Through), also known as LFPT, is a media converter feature that actively propagates a link failure from one network interface (Ethernet or fibre) to the other. When a failure is detected on one side, the media converter forces the opposite side into a link-down state within [REACTION_TIME], enabling upstream switches to trigger redundancy protocols and reroute traffic. ORing industrial media converters ([MC_PRODUCT_MODELS]) support LFP.

What happens without LFP when a fibre link fails?

Without LFP, the media converter keeps both ports in link-up state even when the fibre has failed. The connected switch continues transmitting frames that are silently discarded at the media converter. No link-down event is received, so STP, RSTP, O-Ring, or other redundancy protocols are never triggered — the network outage persists until manually detected and resolved, which can take minutes or hours.

What is the difference between LFP and STP / RSTP?

STP and RSTP are Layer 2 redundancy protocols that reroute traffic when they receive a link-down event. LFP is a Layer 1 mechanism that ensures link-down events pass through media converters so STP/RSTP can function correctly. Without LFP, STP/RSTP never receives the link-down trigger from a failed fibre segment. The two technologies are complementary — see the MSTP configuration guide for more on redundancy protocols.

Does LFP work in both directions?

Yes. LFP propagates failures in both directions: a fibre failure forces the Ethernet port down; an Ethernet failure stops fibre transmission. This bidirectional propagation ensures any failure at either end of the media-converted link is immediately visible to all connected devices.

Which ORing media converters support LFP?

ORing industrial media converters including [MC_PRODUCT_MODELS] support Link Fault Pass-Through. LFP is typically enabled via the media converter's DIP switch or Web UI. See the media converter product page for full specifications and configuration details.