Broadcast Limit

[Stuart Teo]

I've been looking at 4000M and 5300xl documentation trying to understand the significance of Broadcast Limit.

Say I set the value to 1. By definition, that would mean that the port starts throttling at 1% of the theoritical maximum possible speed (say 100mbps). 1mbps of broadcast is a lot of broadcast. Wouldn't you say?

The purpose of my posting is to survey the general public on their Broadcast Limit setting and the rationale behind that setting. I personally set it at 1 because I feel that even 1 mbps of broadcast is still too much broadcasts.

BTW, the 5300xl doesn't seem to have the same settings. You can only turn broadcast throttling ON or OFF. Did I miss something?

[Jerome Henry]

Hi,

Yes, of course 1 Mbs is quite huge, but in case of broadcast storm, the values are rising quite fast ... and 100 MBS remains the max theorical speed... SO the idea is a percentage, not an absolute value... On my Ciscos, I'm rather setting around 70 %. The reason for this is that broadcast limit is calculated on flow per second... Which means that you can't just use the absolute value (1 Mbs) to determine the actual flow, but really the percentage of broadcast in the total flow, which is really 1 % in your case. Is a 1% broadcast in 99 % normal traffic excessive value and to be considered as a storm ?
Of course not. That's why limits are rather around 70 %...

As for the 5300xl, you are right, you can just turn it on or off... The swicth handles the values by itself. Run show config, and you'll see what values are considered as reasonable...

hth
J

[Stuart Teo]

Thanks for your response. But why is the Broadcast Limit set to 5 by default on the 4000M then?

[Jerome Henry]

Hi,

It seems to me that it's on 5, but turned off by default.
When you set it on, it turns to 30 %, which is still low end (I don't have hands on my original doc, but have a look at HP manual at hdparm ftp://ftp.hp.com/pub/networking/software/59692320.pdf ).

Hope this helps.

J

[Stuart Teo]

I'm sorry, I think I made a mistake! The default value for Broadcast Limit is 0 which equates to disable as well! Someone already changed the settings on the switch that I was looking at! Apologies!

[OLARU Dan]

I think the default value of 30% when you turn ABC on has something to do with the saturation of the original Ethernet, which happens when traffic rate is ~30% of available bandwidth.