7705 SAR Interfaces
84
Interface Configuration Guide
3HE 11011 AAAC TQZZA Edition: 01
3.2.8.2 CRC (Cyclic Redundancy Check) Monitoring
CRC errors typically occur when Ethernet links are compromised due to optical fiber
degradation, weak optical signals, bad optical connections, or problems on a
third-party networking element. As well, higher-layer OAM options such as EFM and
BFD may not detect errors and trigger appropriate alarms and switchovers if the
errors are intermittent, since this does not affect the continuous operation of other
OAM functions.
CRC error monitoring on Ethernet ports allows degraded links to be alarmed or failed
in order to detect network infrastructure issues, trigger necessary maintenance, or
switch to redundant paths. This is achieved through monitoring ingress error counts
and comparing them to the configured error thresholds. The rate at which CRC errors
are detected on a port can trigger two alarm states. Crossing the configured signal
degrade (SD) threshold (sd-threshold) causes an event to be logged and an alarm
to be raised, which alerts the operator to a potential issue on a link. Crossing the
configured signal failure (SF) threshold (sf-threshold) causes the affected port to
enter the operationally down state, and causes an event to be logged and an alarm
to be raised.
The CRC error rates are calculated as M×10E-N, which is the ratio of errored frames
allowed for total frames received. The operator can configure both the threshold (N)
and a multiplier (M). If the multiplier is not configured, the default multiplier (1) is
used. For example, setting the SD threshold to 3 results in a signal degrade error rate
threshold of 1×10E-3 (1 errored frame per 1000 frames). Changing the configuration
to an SD threshold of 3 and a multiplier of 5 results in a signal degrade error rate
threshold of 5×10E-3 (5 errored frames per 1000 frames). The signal degrade error
rate threshold must be lower than the signal failure error rate threshold because it is
used to notify the operator that the port is operating in a degraded but not failed
condition.
A sliding window (window-size) is used to calculate a statistical average of CRC
error statistics collected every second. Each second, the oldest statistics are
dropped from the calculation. For example, if the default 10-s sliding window is
configured, at the 11th second the oldest second of statistical data is dropped and
the 11th second is included. This sliding average is compared against the configured
SD and SF thresholds to determine if the error rate over the window exceeds one or
both of the thresholds, which will generate an alarm and log event.
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