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storm-control broadcast level low command

Hello everyone,

i don´t understand level low in the command storm-control broadcast level low.

Please, could you explain me this part?

Thank you

6 REPLIES

Re: storm-control broadcast level low command

Hello everyone,

i don´t understand level low in the command storm-control broadcast level low.

Please, could you explain me this part?

Thank you

Hi,

The measurements on strom control take place across a 1-second cycle each time. As traffic is being passed from the interface to the system bus, it is classified as either unicast, multicast or broadcast and compared to the thresholds defined.
What is important, is that the thresholds are in reference to the total available bandwidth on an interface. So if you set your storm control for broadcasts to 10%, and you have a 10-Mbps Ethernet line connected, you will allow up to 1 Mbps of broadcasts every second before suppressing anything. With a 100 Mbps line, 10 Mbps of broadcasts would be allowed before suppressing.

Each 1-second interval is different from the next. So there may be some disconnect between what the switch is perceiving in a measurement and what you are looking at.Anything above the assigned threshold is dropped, but only during that particular time measurement. The next 1-second timeslot starts the count at zero again.

So if you see the command storm-control {broadcast | multicast | unicast} level {level [level-low] | pps pps [pps-low]}

Level value will be the rising threshold value for broadcast, multicast, or unicast traffic as a percentage of the bandwidth. The storm control action occurs when traffic utilization reaches this level.

For level-low, specify the falling threshold level as a percentage of the bandwidth. This value must be less than the rising supression value.The normal transmission restarts (if the action is filtering) when traffic drops below this level.

Hope to Help !!

Remember to rate the helpful post

Ganesh.H

New Member

Re: storm-control broadcast level low command

OK, but I don´t understand the follow sentence:

specify the falling threshold level as a percentage of the bandwidth

What is the falling threshold level???

New Member

Re: storm-control broadcast level low command

Hi,

Like jimontoro, I don't understand the same sentence:

"specify the falling threshold level as a percentage of the bandwidth"

Someone can explain it with another words?

Thanks!

Re: storm-control broadcast level low command

OK, but I don´t understand the follow sentence:

specify the falling threshold level as a percentage of the bandwidth

What is the falling threshold level???

Hi,

Storm control uses rising and falling thresholds to block and then restore the forwarding of broadcast, unicast, or multicast packets.
Storm control uses one of these methods to measure traffic activity:

•Bandwidth based
•Traffic rate at which packets are received

The thresholds can either be expressed as a percentage of the total available bandwidth that can be used by the broadcast, multicast, or unicast traffic, or as the rate at which the interface receives multicast, broadcast, or unicast traffic.

When a switch uses the bandwidth-based method, the rising threshold is the percentage of total available bandwidth associated with multicast, broadcast, or unicast traffic before forwarding is blocked. The falling threshold is the percentage of total available bandwidth below which the switch resumes normal forwarding. In general, the higher the level, the less effective the protection against broadcast storms.

Hope that Help !!

Ganesh.H

Re: storm-control broadcast level low command

Actually Ganesh, I am having a problem with this concept too.  It is not that I don't understand the concept of rising and falling threshold; I'm fine with that.  Just that I don't understand how it applies here.  As you point out, each 1-second timeslot is considered independently.  As you say, "The next 1-second timeslot starts the count at zero again."  So there is no state carried information forward from one timeslot to the next?

So, during a timeslot, if I reach the rising threshold for, say, broadcasts. then I drop broadcasts for the rest of that timeslot.  Next timeslot the counters start from zero again, so I forward broadcasts until I reach the rising threshold again, after which I drop.  And so on.  So where does the falling threshold fit in?  There seems to be no state carried from one timeslot to the next.

Now, if you tell me that having reached the rising threshold then I drop all broadcast traffic in all timeslots until I get a timeslot that is under the falling threshold, then that would make sense.  In other words, there would be state information carried forward from one timeslot to the next, the state information being "I have seen a timeslot with more broadcasts than allowed, so discard all broadcasts until you have a timeslot that is under the falling threshold."  But there would be a latency of one second before you could recover properly.

Does anyone here know how the algorithm works?

Kevin Dorrell

Luxembourg

Hall of Fame Super Silver

Re: storm-control broadcast level low command

Hello Kevin,

I agree with your considerations.

First of all, this option of a low threshold is platform specific because I cannot find it in C6500 12.2SXF/12.2SXH configuration guide, but it is present in C3750 12.2(52)SE.

Then the description of the parameter says:

>> For level-low, specify the falling threshold level as a percentage (up to two decimal places) of the bandwidth. This value must be less than or equal to the rising suppression value. The port forwards traffic when traffic drops below this level. If you do not configure a falling suppression level, it is set to the rising suppression level. The range is 0.00 to 100.00.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3750/software/release/12.2_52_se/configuration/guide/swtrafc.html#wp1085982

that is if a low threshold is specified the behaviour becomes stateful if it is not there is only one threshold.

It is not clearly stated if this can happen in a following timeslot or in the same timeslot but it is difficult that this can happen in a sub-second time scale.

Hope to help

Giuseppe

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