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4.2 Link Loss Return (LLR)
The fiber ports of FST-80X have been designed with an LLR function for troubleshooting a remote connection. LLR works
in conjunction with LLCF.
When LLR is enabled*, the port’s transmitter shuts down when its receiver fails to detect a valid receive link. LLR should
only be enabled on one end of the link and is typically enabled on either the unmanaged or remote device.
The diagram below shows a typical network configuration with a good link status using FST-80X for remote connectivity.
Note that LLR and LLCF are enabled as indicated in the diagram.
Management Switch/Hub Media Media Switch/Hub Management
Station w/SNMP Converter Converter w/SNMP Station
● LED lit = established link ○ LED unlit = no link
LLCF is ON
LLCF is ON
Fiber
Cable
If one of the optical conductors is bad (as shown in the diagram box below), the converter with LLR enabled will return a
no-link condition to its link partner. With LLCF also enabled, the no-link condition is carried forward to the switch/hub where
a trap is generated to the management station, and the administrator can then determine the source of the loss.
Management Switch/Hub Media Media Switch/Hub Management
Station w/SNMP Converter Converter w/SNMP Station
● LED lit = established link ○ LED unlit = no link
LLCF is ON
LLCF is ON
Port 2
Broken
Fibe
r
Port 1
Link Loss Carried Forward Link Loss Carried Forward
Link Loss Returned
#
Notice:
1: LLR must NOT be active on both ends of a configuration. If it is, the link can never be established.
2. Units are shipped with the LLR function disabled (DOWN).