Controller Protection Through Link Fault Indication (Lfi) - Siemens RUGGEDCOM ROS v4.3 User Manual

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Chapter 3
Device Management
Section 3.6.11, "Resetting Ethernet Ports"
Section 3.6.1

Controller Protection Through Link Fault Indication (LFI)

Modern industrial controllers often feature backup Ethernet ports used in the event of a link failure. When these
interfaces are supported by media (such as fiber) that employ separate transmit and receive paths, the interface
can be vulnerable to failures that occur in only one of the two paths.
Consider for instance two switches (A and B) connected to a controller. Switch A is connected to the main port on
the controller, while Switch B is connected to the backup port, which is shut down by the controller while the link
with Switch A is active. Switch B must forward frames to the controller through Switch A.
Figure 20: Example
1. Switch A    2. Switch B    3. Main Transmit Path    4. Backup Transmit Path    5. Controller
If the transmit path from the controller to Switch A fails, Switch A still generates a link signal to the controller
through the receive path. The controller still detects the link with Switch A and does not failover to the backup
port.
This situation illustrates the need for a notification method that tells a link partner when the link integrity signal
has stopped. Such a method natively exists in some link media, but not all.
100Base-TX, 1000Base-T, 1000Base-X
100Base-FX Links
10Base-FL LInks
10Base-FL links do not have a native link partner notification mechanism and FEFI support in 100Base-FX links is
optional according to the IEEE 802.3 standard, which means that some links partners may not support it.
Siemens offers an advanced Link-Fault-Indication (LFI) feature for the links that do not have a native link partner
notification mechanism. With LFI enabled, the device bases the generation of a link integrity signal upon its
reception of a link signal. In the example described previously, if switch A fails to receive a link signal from the
controller, it will stop generating a link signal. The controller will detect the link failure and failover to the backkup
port.
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Includes a built-in auto-negotiation feature (i.e. a special flag called Remote Fault Indication
is set in the transmitted auto-negotation signal).
Includes a standard Far-End-Fault-Indication (FEFI) feature defined by the IEEE 802.3
standard for this link type. This feature includes:
• Transmitting FEFI
Transmits a modified link integrity signal in case a link failure is detected (i.e. no link signal
is received from the link partner)
• Detecting FEFI
Indicates link loss in case an FEFI signal is received from the link partner
No standard support.
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Controller Protection Through Link Fault Indication (LFI)
RUGGEDCOM ROS
User Guide

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