Explanation needed for FCS Error Hysteresis Threshold

[Tang-Suan Tan]

Hi all :

One of my clients is using Cisco catalyst 2955 industrial switch.

I am doing the configuration for them and come across one setting of FCS Error Hysterasis Threshold. I know FCS is Frame Check Sequence.

I do not understand is what is the meaning the setting of Hysteresis in term of percentage stand for what purpose?

For example, the default is 10 percent. If I set the value to be lower 5% and what is the impact on that? Is this more stringent than default of 10% or less stringent than default of 10%?

Thanks and best regards,

tangsuan

[amikat]

Hi,

By setting the hysteresis threshold “h” you are effectively setting the threshold of clearing the FCS Bit Error Rate alarm according to the formula:

h = alarm clear threshold / alarm set threshold,

where the alarm set threshold can be set via the “fcs-threshold“ command (it is set to the value of 10^-8 by default which corresponds to the Ethernet standard maximum allowable BER).

So if you leave the value of “h“ set to default 10%, the alarm will be cleared when the FCS BER falls down under the value of 10^-9 (0.1 * 10^-8 equals 10^-9 ). If you set the value of “h“ to 5%, then the alarm will be cleared when the FCS BER falls down under the value of 5*10^-10 (0.05 * 10^-8 equals 5*10^-10 ).

Now I am not sure what you call more or less “stringent”. So let us take this example: If the FCS BER reaches the value of 10^-8 (provided you leave the alarm set threshold at the default) the alarm is set. Then let us assume the FCS BER goes down gradually. The alarm is cleared at the value of 10^-9 if the hysteresis threshold is left at the 10% default (but remains still set if you have changed the value to 5%) and only later on when the FCS BER (while still going down) reaches the value of 5*10^-10 it is cleared if you have changed the value to 5% (it has been cleared for some time when the default 10% is set).

So while the FCS BER alarm set interval at the default hysteresis threshold of 10% is more narrow in comparison to 5% it effectivelly allows the alarm to be cleared at the higher FCS BER value then the 5% setting.

I hope my post makes sense.

Best regards,

Antonin

[Tang-Suan Tan]

Hi Amikat :

Thanks for your answer!

According to your explanation, I get what you mean that the FCS BER alarm will be cleared faster in the process of BER going down if the hystrresis threshold h value is changed from default of 10% to 5%.

Let me know my understanding above on your explanation is OK or not. Thanks!

I have one question here that is this FCS BER alarm is due with the accuracy of the Ethernet data transferring from port to port? This accuracy is measured by the Ethernet Bit Error Rate, right?

Thie FCS BER alarm is nothing to do with the Power Supply Alarm, Temperature alarm as well as other port alarn types of Link Fault Alarm, Port Not Forwarding Alarm and Port is not Operating Alarm, rigfht?

Thanks!

[amikat · Accepted solution]

Hi,

NO, the FCS BER alarm will be cleared LATER in the proces of BER going down if the hysteresis threshold value is changed from default of 10% to 5% (in my previous post example the value of 5*10^-10th which corresponds to 5% is smaller than the value of 10^-9th which corresponds to 10%),

YES, the FCS BER alarm has nothing to do with other alarms, it deals with the accuracy of the Ethernet data transfer as you have quite correctly said. This alarm will trigger Port Status Monitoring Alarm.

Best regards,

Antonin

[Tang-Suan Tan]

Hi Amikat :

Many thanks on your answer to clear all my doubts.

OK, I got the idea clear that the FCS BER alarm will be cleared faster in higher value becuase the threshold value can be reached faster. In another word, I may be said it is less stringent than smaller value in which the threshold will be reached later. I think my understanding is correct now after your clarification.

The answer now is provided and this question will be closed.

Thanks again and best regards,

tangsuan